ArticlePDF Available

Invasive Aquatic Vegetation Management in the Sacramento–San Joaquin River Delta: Status and Recommendations

  • USDA-ARS (at University of California, Davis)
  • Delta Stewardship Council
  • California Department of Fish and Wildlife, Stockton, United States

Abstract and Figures

Widespread growth of invasive aquatic vegetation is a major stressor to the Sacramento-San Joaquin River Delta, a region of significant recreational, economic, and ecological importance. Total invaded area in the Delta is increasing, with the risk of new invasions a continual threat. However, invasive aquatic vegetation in the Delta remains an elusive ecosystem management challenge despite decades of directed scientific research and prioritized policy recognition. In this paper, we summarize the current state of knowledge of the history, status, and potential future directions for coordinated research, management actions, and policy, based on topics discussed at the symposium held on invasive aquatic vegetation on September 15, 2015. Remote sensing technology; mechanical, chemical, and biological control; as well as community science networks have all been shown to be effective management tools, but overall effectiveness has been hindered by complex regulatory structure, the lack of a consistent monitoring program, regulations that restrict treatments in space and time, and funding cuts. In addition, new management options depend on continued research and development of new active ingredients for chemical control as well as testing of biological control agents. The ongoing development and implementation of new strategies for adaptive, integrated management of aquatic weeds-using currently-available management tools, new knowledge derived from remote sensing and plant growth models, and an area-wide, ecosystem-based approach-is showing promise to achieve improved management outcomes and enhance protection of the Delta's water resources.
Content may be subject to copyright.
UC Davis
San Francisco Estuary and Watershed Science
Invasive Aquatic Vegetation Management in the Sacramento–San Joaquin River Delta:
Status and Recommendations
San Francisco Estuary and Watershed Science, 15(4)
Ta, Jenny
Anderson, Lars W.J.
Christman, Maggie A.
et al.
Publication Date
CC BY 4.0
Peer reviewed Powered by the California Digital Library
University of California
Widespread growth of invasive aquatic vegetation
is a major stressor to the Sacramento–San Joaquin
River Delta, a region of significant recreational,
economic, and ecological importance. Total invaded
area in the Delta is increasing, with the risk of new
invasions a continual threat. However, invasive
aquatic vegetation in the Delta remains an elusive
ecosystem management challenge despite decades
of directed scientific research and prioritized policy
recognition. In this paper, we summarize the current
state of knowledge of the history, status, and
potential future directions for coordinated research,
management actions, and policy, based on topics
discussed at the symposium held on invasive aquatic
vegetation on September 15, 2015. Remote sensing
technology; mechanical, chemical, and biological
control; as well as community science networks
have all been shown to be effective management
tools, but overall effectiveness has been hindered by
complex regulatory structure, the lack of a consistent
monitoring program, regulations that restrict
treatments in space and time, and funding cuts.
In addition, new management options depend on
continued research and development of new active
ingredients for chemical control as well as testing of
biological control agents. The ongoing development
and implementation of new strategies for adaptive,
integrated management of aquatic weeds—using
currently-available management tools, new
knowledge derived from remote sensing and plant
growth models, and an area-wide, ecosystem-based
approach—is showing promise to achieve improved
management outcomes and enhance protection of the
Delta’s water resources.
Aquatic vegetation, invasive species, invasive species
management, ecosystem engineers, remote sensing,
biological control, Sacramento–San Joaquin River
Invasive Aquatic Vegetation Management
in the Sacramento–San Joaquin River Delta:
Status and Recommendations
Jenny Ta *, 1, Lars W. J. Anderson
2, Mairgareth A. Christman
3, Shruti Khanna
4, David Kratville
5, John D. Madsen
6, Patrick J. Moran
and Joshua H. Viers
Volume 15, Issue 4 | Article 5
* Corresponding author:
1 School of Engineering, University of California, Merced
Merced, CA 95343 USA
2 Waterweed Solutions
Point Reyes Station, CA 94937 USA
3 Delta Science Program, Delta Stewardship Council
Sacramento, CA 95814 USA
4 University of California, Davis
Davis, CA 95616 USA
5 Integrated Pest Control Branch
California Department of Food and Agriculture
Sacramento, CA 95832 USA
6 Exotic and Invasive Weed Research Unit,
U.S. Department of Agriculture–Agricultural Research Service and
Department of Plant Sciences, University of California, Davis
Davis, CA 95616 USA
7 Department of Civil and Environmental Engineering,
University of California, Merced
Merced, CA 95343 USA
The confluence of the Sacramento and San Joaquin
rivers in northern California serves as a vital crossing
point for water conveyances and shipping channels,
and as habitat for numerous aquatic species and
waterfowl along the Pacific Flyway. From early
construction of levied islands, to the export of
water to central and southern parts of the state, to
alterations in the natural flow of tributaries, more
than a century of human alteration has dramatically
changed the Delta (Whipple et al. 2012). Among
the more recent critical changes is the proliferation
of invasive aquatic vegetation. The San Francisco
Estuary, including the Sacramento–San Joaquin
River Delta, is known to be the primary gateway
for biological invasion in the western United States,
and has been described as one of the most invaded
estuaries in the world (Cohen and Carlton 1998).
If not successfully managed, invasive aquatic
macrophytes are one of the potential dynamic drivers
that could upend Delta recovery. Because of the
ecological and economic significance of the Delta, it
is imperative that we strive to understand the effects
of invasive aquatic vegetation and take appropriate
management action to mitigate these effects.
The Delta Interagency Invasive Species Coordination
Team (DIISC), an interagency working group made
up of representatives from agencies involved in
detecting, preventing, and managing invasive
species and invaded habitats in the Sacramento-San
Joaquin Delta, is working to improve collaboration,
coordination, and communication for invasive species
management issues. Members of this collaborative
process have identified the following key topics
regarding invasive aquatic vegetation.
Environmental drivers of infestations
Understanding of spatial dynamics by mapping
Regulatory and governance hurdles
Risk assessment
Options for control
Lessons from other regions such as Florida
These topics were the focus of a day-long invasive
aquatic weed symposium at the University of
California, Davis was held on September 15, 2015,
with the goal of summarizing current understanding
of these issues identified by agency leaders and
developing recommendations on next steps for
aquatic weed management in the Delta (DSC 2015).
The symposium focused primarily on four taxa:
hydrilla (Hydrilla verticillata), Brazilian waterweed
(Egeria densa), water hyacinth (Eichhornia crassipes),
and water primrose (Ludwigia spp.). Participants
of the symposium included representatives from
the Delta Science Program, U.S. Department of
Agriculture (USDA) Agricultural Research Service,
California Department of Fish and Wildlife, California
Department of Food and Agriculture, National
Aeronautics and Space Administration, University
of California–Davis, Smithsonian Environmental
Research Center, Florida Fish and Wildlife
Conservation Commission, Anaerobe Systems, and
the U.S. Army Engineer Research and Development
Center. Presentation videos have been made available
online by the Delta Stewardship Council (Delta
Stewardship Council 2015). This article summarizes
the current status of the key topics symposium
organizers identified.
Environmental Drivers
Aquatic vegetation or macrophytes, including
emergent, floating, or submersed forms, grow in the
littoral zone of water bodies. Generally, emergent
macrophytes are found in shallow areas, with
submersed plants in deeper water, and free-floating
plants throughout the water surface. Floating and
emergent plants have unlimited water, unimpeded
access to light, and an atmospheric source of carbon
dioxide. In contrast, submersed plants are subject to
much lower levels of light and carbon dioxide. Water
clarity and the maximum depth of light penetration
typically drive submersed plant distribution and
In the Delta, both water depth and high turbidity
have been correlated with distribution of submersed
vegetation, particularly Egeria densa (Durand et
al. 2016). Water velocity is also an important
environmental factor. Floating aquatic macrophytes
grow in slower- moving water compared to
submersed aquatic macrophytes, which grow in
slightly swifter water flows and tend to grow more
rapidly in flowing water. The twice-daily tidal flows
in the Delta thus provide ideal conditions for rapid
growth of submersed aquatic plants. Rooted aquatic
macrophytes obtain their nutrients mainly from
sediment but can utilize nutrients in the open water
as well. High levels of nutrients, particularly nitrogen
and phosphorus, have been suggested as facilitating
the recent expansion of aquatic vegetation in the
Delta, but very little evidence exists to conclusively
determine the role of nutrients in driving this growth
(Boyer and Sutula 2015; Dahm et al. 2016).
Major Aquatic Macrophytes in Sacramento–San
Joaquin Delta
Table 1 lists the major aquatic macrophytes observed
in the Delta. This section summarizes information on
four taxa: hydrilla (Hydrilla verticillata), Brazilian
waterweed (Egeria densa), water hyacinth (Eichhornia
crassipes), and water primrose (Ludwigia spp.)
(Figure 1), with a few additional key species such as
giant reed (Arundo donax) and Eurasian watermilfoil
(Myriophyllum spicatum). Of these species, all occur
in California, but only hydrilla is not currently found
in the Delta.
Hydrilla is a non-native submersed aquatic
macrophyte that can fill the water column up
to 20 feet in depth. It comes in monoecious and
dioecious forms, and can be identified by five heavily
Figure 1 Four invasive aquatic macrophytes, clockwise from upper left: Eichhornia crassipes (Photo credit: Wouter Hagens, public domain);
Ludwigia peploides (Gabriel Bell, public domain); Egeria densa (Photo credit: Lara Gudmundsdottir, CC BY-SA 4.0); Hydrilla verticillata (Photo
credit: Andrew Benassi, public domain).
serrated leaves in a whorl, with stipules (small spines)
on the mid-vein. Hydrilla is adapted to climates with
alternating rainy and dry seasons, and produces tubers
5- to 25-cm deep in the sediment, which allows them
to survive dry seasons. These tubers can last for over
4 years and regrow with rain (Van and Steward 1990).
Hydrilla can also disperse vegetatively through turions,
or buds, that fall off and disperse in the water column.
It also spreads through stem fragments (two to three
nodes can start a new plant). Hydrilla has perennial
root crowns that can over-winter and re-grow rapidly
in the spring. Hydrilla was first discovered in Imperial
County, in southern California, and in a Yuba City
park lake. Models have shown that California is
very suitable for growth of Hydrilla (Barnes et al.
2014), and lessons learned from other systems (e.g.,
Florida) indicate that Hydrilla is a significant threat to
most if not all the freshwater California ecosystems,
and would be particularly problematic if it were to
successfully establish in the Delta.
A major challenge associated with Hydrilla control
and eradication is the persistence of its abundant
propagules: turions and tubers. To date, the most
effective technique for targeting tubers is a modified
gold dredge, capable of removing sediment, which is
then screened for plant fragments. The combination
of two types of chemical applications has proven
effective in California. Contact herbicides (copper,
endothall) are used to kill or suppress actively
growing plants to stop propagule production and
fragmentation. The systemic herbicide fluridone is
utilized to wear down the tuber population by killing
existing Hydrilla as well as the newly sprouted
tubers. Seasonal applications must be continued
for 5 to 7 years to ensure that no plants are able
produce new propagules of any kind. Historically,
fumigation of drawn down (de-watered) water
bodies was very effective at killing tubers in the soil.
However, currently no fumigants are registered for
use in California aquatic systems. Cultural controls,
such as pond liners and cement lining have been
used to isolate the tubers from contact with the water
body. Broad-spectrum biological controls have also
been used in Imperial County, where triploid grass
carp were released to consume all the submersed
Table 1 Common floating and submersed aquatic macrophytes found in the Delta (modified from Boyer and Sutula 2015)
Species (common name) Invasive? Type
Egeria densa (Brazilian waterweed) invasive submersed
Myriophyllum aquaticum (Parrot feather) invasive submersed/emergent
Myriophyllum spicatum (Eurasian watermilfoil) invasive submersed
Potamogeton crispus (curlyleaf pondweed) invasive submersed
Cabomba caroliniana (Carolina fanwort) invasive submersed
Stuckenia pectinata (sago pondweed) native submersed
Ceratophyllum demersum (coontail) native submersed
Potamogeton nodosus (American pondweed) native submersed
Elodea canadensis (common waterweed) native submersed
Eichhornia crassipes (water hyacinth) invasive floating
Limnobium laevigatum (South American sponge plant) invasive floating
Ludwigia hexapetala (Uraguayan water primrose) invasive floating
Ludwigia peploides (water primrose) invasive floating
Hydrocotyle umbellata (pennywort) native floating
Azolla (mosquito fern) native floating
Lemna spp (duckweed) native floating
Potamogeton foliosus (leafy pondweed) native submersed
Ludwigia palustris (water purslane) native floating
Ruppia maritima (widgeongrass) native submersed
Brazilian waterweed is an invasive submersed aquatic
macrophyte native to Brazil and Argentina that
was introduced to the Delta in the 1960s, reaching
problem levels in the 1990s (Cook and Urmi–
Konig 1984; Foschi and Liu 2002, unreferenced,
see “Notes"). Its presence increases sedimentation
and reduces water velocity, which helps support its
expansion (Yarrow et al. 2009). Mats of Brazilian
waterweed grow up to 13 feet deep in clear channels,
reducing oxygen levels during the night and
hindering the movement of fish between subtidal
open water and tidal wetlands. Egeria’s relatively
low salinity tolerance hinders its competitive ability
in the western Delta and Suisun Marsh relative to
Stuckenia pectinata, a native submersed species that
tolerates moderately saline (brackish) conditions
(Borgnis and Boyer 2015).
Besides submersed aquatic macrophytes, the Delta
also has abundant floating aquatic macrophytes. The
most abundant invasive floating aquatic macrophyte
is water hyacinth, which was first documented in the
Sacramento River in 1904 (Finlayson 1983; Toft et al.
2003). Water hyacinth has one of the highest growth
rates of any vascular macrophyte, and studies have
found a doubling time that ranges from 10 to 60 days
(Téllez et al. 2008). It is a freshwater plant that cannot
tolerate more than about 3% salinity (Haller et al.
1974). It has a highly plastic (variable) morphology
with two to three phenotypic forms. Mats of water
hyacinth depress oxygen levels (Penfound and Earle
1948; Villamagna and Murphy 2010) and have been
shown to support different invertebrate communities
compared to native species such as pennywort
(Hydrocotyle umbellata) (Toft et al. 2003).
Another more recent, invasive, rooted emergent
aquatic macrophyte found in the Delta is water
primrose. Ludwigia peploides was found in the Delta
in 1949, and in California as early as 1916 (Light
et al. 2005). It has been considered invasive during
the past 5 years because of its rapid and dramatic
increase in coverage in the Delta (Khanna et al. 2015,
unreferenced, see “Notes"). Water primrose grows in
slow-moving or still water and is also highly plastic
in morphology, able to grow erect on water and in
creeping form on land. New information suggests
there are no native Ludwigia species in the Delta, but
rather only three species, all of which originate from
South America; L. peploides, L. hexapetala, and L.
grandiflora (Grewell et al. 2016). Purported ecosystem
effects of water primrose are similar to those of water
hyacinth, including reduced oxygen levels, hindered
fish movement, and mosquito breeding habitat. A
study of Ludwigia hexapetala and L. grandiflora in
California freshwater wetlands found that because of
low seedling recruitment, management should focus
on the clonal vegetative dispersal of these species
(Okada et al. 2009).
Based on 2014 remote sensing data, the Delta
was covered by 7,550 acres of submersed aquatic
vegetation and 3,810 acres of floating aquatic
vegetation (Khanna et al. 2015; SFEP 2015). Total
wetted area of the Delta is on the order of 60,000
acres, which fluctuates with tides and seasons. From
2008 to 2014, the total invaded area of submersed
and floating aquatic vegetation in the Delta increased
by 60%, from 7,100 acres to 11,360 acres. Submersed
aquatic vegetation composition showed little change
and remains dominated by Brazilian waterweed.
From 2004 to 2008, the co-dominant floating aquatic
species were pennywort, water hyacinth, and water
primrose. In 2014, the co-dominant species were
water primrose and water hyacinth, with coverage
of 1,050 and 3,000 acres, respectively. More recent
2014 and 2015 remote sensing data for the Delta
include areas that are proposed for restoration,
such as Prospect Island. Taken together, these data
show increases in submersed aquatic vegetation
from 11,876 acres in 2014 to 13,950 acres in 2015.
Floating aquatic vegetation decreased from 4,156
acres in 2014 to 3,457 acres in 2015. Coverage of
water hyacinth dropped from 3,151 acres in 2014
to 1,072 acres in 2015. In contrast, water primrose
species increased from 1,005 acres in 2014 to 1,267
acres in 2015. The increasing rate of spread of water
primrose in the northwest Delta is becoming a major
concern, and the California Department of Parks
and Recreation, Division of Boating and Waterways
(CDBW) recently (2016) authorized chemical control
for this species using the same tools as for water
The following section describes general trends in
spatial distribution of floating aquatic macrophytes
in the Delta. Water primrose dominates in the
northwest, such as at Liberty and Prospect islands. It
is also present in Sherman Island, an area that is near
brackish water. Water primrose is not a true floating
plant like water hyacinth, but is instead a creeping
emergent with floating stems and highly variable
leaf morphology, better adapted to high flow, tidal
and wave action, and higher salinity than water
hyacinth (Ustin et al. 2015b). Though water primrose
is generally found everywhere in the Delta where
water hyacinth exists, the central and southern Delta
is dominated by water hyacinth. In the southeast
and east Delta, water hyacinth and water primrose
are co-dominant (Ustin et al. 2015b). Water primrose
occurs at the emergent marsh edge along shorelines,
while floating mats of water hyacinth tend to be
found in deeper water. The northern part of the
flooded Liberty Island in the Delta has experienced
an increase in emergent vegetation every year. Based
on available remote sensing data, water primrose
coverage at the island has increased every year.
Factors Facilitating Increase in Invasive Aquatic
Macrophytes in the Delta
Further research is needed to better understand the
drivers of increasing invasive aquatic macrophytes in
the Delta. Here, we highlight a few issues that may
have contributed to the spread of aquatic invasive
plants. Although state agencies play a critical role
in management of invasive vegetation, the loss of
funding in recent years has limited their ability
to take management actions. For example, budget
cuts in 2010 and 2011 eliminated nearly $3 million
from the terrestrial and aquatic weed programs.
In addition, between fiscal years 2010/2011 to
2012/2013 the California Department of Food and
Agriculture (CDFA) Weed Biocontrol funding was
cut from a little more than $1 million to zero.
Furthermore, water primrose, a key species that has
greatly expanded its coverage over the past 5 to 7
years, has only recently (2016) been authorized for
treatment with herbicides.
Regulations restrict when and where mechanical
control occurs, when and where herbicides can be
used, and which herbicides can be used. The fish
passage protocol restricts treatments to one-half to
one-third of the water hyacinth mat at a time, which
is problematic since the plant doubles in 7 to 10
days. Lawsuits prevented all control of invasives for
two years in 2000 and 2001, which led to a huge
increase in water hyacinth cover.
The continuing drought and resulting increase in
shallow water habitat may also provide more suitable
habitat for invasive weeds. Invasive weeds such
as water hyacinth are vulnerable to frosty nights
(Penfound and Earle 1948) but mild winters in the
past few years, along with the lack of large storms
and fast flows, have favored the persistence and
spread of submersed and floating aquatic vegetation.
High nutrient levels, while perhaps not driving
growth, likely at least support the high growth rates
(Dahm et al. 2016). Finally, invasive plants act as
ecosystem engineers and, through positive feedbacks
such as low velocity, sediment deposition, and lower
turbidity, support their own spread (Shih and Rahi
1981; Petticrew and Kalff 1992; Green 2005).
Future Implications for the Delta
Recent (2015) increased funding for CDBW aquatic
invasive plant control programs, recent (2015–2016)
approvals of two additional species for treatment,
and the implementation, since 2014, of the USDA–
ARS (Agricultural Research Service)-funded Delta
Region Area-wide Aquatic Weed Project (DRAAWP)
should result in better implementation of integrated,
adaptive management of invasive aquatic weeds in
the Delta. It is important to consider, however, that
the reduction of invasive plant coverage does not
necessarily favor native species in open water. For
example, Brazilian waterweed and water hyacinth
are ecosystem engineers which modify their
environment to benefit their growth. When Brazilian
waterweed is removed, other non-native and native
submersed aquatic vegetation patches may replace
it. The removal of water hyacinth in the Delta can
unexpectedly favor the spread of submersed aquatic
plants like Brazilian waterweed (Khanna et al. 2011a).
All these examples indicate that a return to a pre-
invasion state of the system is unlikely, despite
invasive plant removal, as evidenced in the state
of Florida, where active management of an even
broader range of non-native aquatic macrophytes
has occurred for many more decades than in the
Delta. Sea level rise may push more saline water
into the Delta and reduce the cover of freshwater
invasive species, but warmer temperatures may favor
the growth of tropical invasive plants. Management
of invasive aquatic macrophytes may need to take
advantage of new climatic and environmental
Management of invasive aquatic macrophytes in the
state of California is conducted through a number
of different federal and state agency programs in
coordination with non-profit organizations and
research universities. The lead department for the
control of noxious weeds state-wide is the CDFA
(CDFA [date unknown]). The CDBW is currently
the lead agency for cooperating and administering
aquatic plant management programs in the
Sacramento–San Joaquin Delta, its tributaries, and
the Suisun Marsh. Legislation (AB 763 [2013], which
added Section 64.5 to the Harbors and Navigation
Code), requires the CDBW to consult other agencies
to add aquatic weed species to the management
program and determine management priorities
through an interagency process.
As the lead agency in the state for noxious weed
control, the CDFA cooperates with federal, state,
county, and local agencies; county agricultural
commissioners; and private entities to prevent
the spread of invasive aquatic weeds (CDFA [date
unknown]). The main focus of the CDFA’s Hydrilla
Eradication Program is to protect state water
resources from invasive aquatic weed infestations,
primarily Hydrilla. The program surveys waterways
and implements a zero-tolerance policy, with the
explicit goal of eradicating Hydrilla once detected.
Hydrilla is the only aquatic macrophyte mandated
by state law to be eradicated. As of September
2015, Hydrilla has not been found in the Delta,
unlike Brazilian waterweed, which is found in
high abundance throughout the Delta. The Hydrilla
Eradication Program plays a critical role in early
detection and rapid response, and is important for
preventing the introduction and establishment of
Hydrilla in the Delta. However, this program in recent
years has faced funding cuts that limit detection
surveys, thereby increasing the risk of Hydrilla
establishment. Active projects to eradicate Hydrilla
are in Clear Lake, Yuba County, Nevada County, and
at the Sacramento River near Redding. Because of the
long-lived tuber “bank,” herbicide applications need
to be maintained for many years before eradication
is approached. Research needs for the eradication
of Hydrilla are: a replacement for fumigation,
DNA tracing of populations and introductions,
management strategies that prevent development of
herbicide resistance, and studies on how to maintain
herbicide contact time with water flow and tidal
influence. To maintain the continued success of
Hydrilla eradication, management alternatives for
fumigation need to be developed.
The California Department of Fish and Wildlife
(CDFW) has adopted the U.S. Aquatic Weed Risk
Assessment (WRA) model to estimate the “risk
of invasion” posed by introduced aquatic species
(Gordon et al. 2012) and to approve their treatment.
The review process consists of 36 questions related
to the ecology, competitive ability, dispersal modes,
reproductive capacity, potential effects, management
resistance, and invasion history of a species. The
model produces a cumulative numerical score; a
score under 31 indicates non-invasiveness, and scores
above 39 indicate major invasiveness. The CDBW
has requested that five species be assessed (as of
September 2015):
1. curlyleaf pondweed (Potamogeton crispus) with
a U.S. Aquatic WRA score of 66, authorized for
treatment in 2015 using the same tools as for
Brazilian waterweed;
2. Eurasian watermilfoil, with a tentative score of
3. water primrose, authorized for treatment in 2016
using the same tools as for water hyacinth;
4. coontail (Ceratophyllum demersum); and
5. Carolina fanwort (Cabomba caroliniana), in queue
for assessment.
Interagency Coordination
Management of other invasive aquatic macrophytes
in California is conducted by several different
agencies, leading to complexity in coordinating
management actions. The current state of how
invasive aquatic macrophytes are managed needs to
be more clearly understood, as do the authorities,
roles, and goals of the various agencies (e.g., CDFA,
CDBW, CDFW, etc.). In contrast to California’s multi-
agency approach, the state of Florida has a single
agency, the Florida Fish and Wildlife Conservation
Commission Aquatic Plant Management Section,
which funds, designs, coordinates, and contracts
invasive non-native aquatic plant control efforts
in the state’s 1.25 million acres of public waters
(FDFWCC 2012).
In response to the large number of agencies and
organizations involved in invasive plant management
in California, inter-agency partnerships such as the
Delta Interagency Invasive Species Coordination
Team have emerged to promote greater coordination
and information sharing. In 2016, the Interagency
Ecological Program (IEP) formed the Aquatic
Vegetation Project Work Team (PWT). The team
is planning to develop conceptual models that
describe aquatic plant species in the Delta, along
with special studies and monitoring strategies to
determine the effects of management activities on
the aquatic ecosystem. In addition, there is a new
IEP Management, Analysis, and Synthesis Team
(IEP–MAST) project focused on developing effective
control strategies in the Delta to promote Delta Smelt
resilience, and to test the water quality effects of
herbicides and invasive-weed removal.
Delta Region Area-wide Aquatic Weed Project
Currently in its third year, the DRAAWP aims to
conduct area-wide adaptive, integrated management
of water hyacinth, Brazilian waterweed, and giant
reed in the Delta through science-based control
strategies. This approach includes prioritization and
optimization of management actions. The strategies
and technology developed through DRAAWP will
have applications for control of other aquatic
invasive weeds. Expected outcomes are decreased
control costs, increased water conveyance efficiency,
decreased economic damage to navigation, improved
suppression of mosquito populations near aquatic
weed infestations, and increased wetland restoration
opportunities. The project aims to improve long-
term sustainable management of aquatic weeds by
providing agencies with the information necessary
to optimize control methods for the seasonal and
spatial targeting of aquatic weed populations. The
project has enabled federal, state, regional, and
local agencies and stakeholders to work together
to improve control outcomes and reduce damage. The
project funds implementation of improved integrated,
adaptive management of water hyacinth, Brazilian
waterweed, and giant reed by supporting the CDBW’s
environmental monitoring while they conduct chemical
and mechanical control of water hyacinth and Brazilian
waterweed, and by supporting the Sacramento–San
Joaquin Delta Conservancy’s efforts to control giant
reed using mostly chemical herbicide application
methods. The project also augments the USDA–ARS
and cooperators’ implementation of biological control
of water hyacinth and giant reed, using insects recently
permitted for field release. The DRAAWP supports
assessment of aquatic weed populations and of control
success using remote sensing tools developed by NASA–
Ames Research Center and on-water assessment of
both aquatic weed populations and key environmental
variables such as dissolved oxygen. The DRAAWP
supports research by the USDA–ARS and UC Davis
scientists to test new herbicides and integrated control
methods, and to determine seasonal aquatic weed
growth cycles in relation to control. Other research goals
supported by the project include the following:
1. Development, by NASA and UC Davis scientists, of a
USDA–ARS Soil–Water Assessment Tool (SWAT) and
GIS-based model to determine the effects of land
use on water quality with relevance to aquatic weed
2. Determination by UC Davis scientists and local
mosquito vector control districts of associations
between aquatic weeds and populations of mosquito
larvae; and
3. Development, by UC Davis, of a “bioeconomic”
model of costs and damage associated with aquatic
weed invasions in the Delta, and an estimation of
project benefits.
Remote Sensing Technology
The standard approach to invasive aquatic macrophyte
management has generally been guided by the doctrine
of early detection and rapid response, for which remote
sensing is the tool of choice (Hestir et al. 2008), though
it is limited by the frequency of data acquisition. In
the Delta, this approach has provided the majority of
the data available for species distribution and coverage
over the last 10 to 15 years. It is important to keep
in mind that remote sensing is best complemented
with ground-based surveys, in particular to identify
species of submersed plants. Hyperspectral remote
sensing, an imaging spectroscopy technique that
produces data sets with hundreds of spectral bands of
narrow bandwidth (5–15 nm), has been used to map
invasive aquatic macrophytes in the Delta (Hestir et
al. 2008; Khanna et al. 2011b; Ustin et al. 2015b).
Consistent hyperspectral remote sensing data are
needed to map and monitor the spread of invasive
species. Hyperspectral data is particularly useful
because of the large number of bands available.
For example, HyMap data consists of 126 bands
(400–2400 nm), and AVIRIS–NG Airborne Visible and
InfraRed Imaging Spectrometer Next Generation,
NASA JPL’s high-resolution platform data consists of
432 bands (350–2500 nm). Hyperspectral data enable
the measurement of biophysiological characteristics
of different aquatic macrophyte species, which cannot
be measured using the few broad bands available
in current satellite sensors. Five consecutive years
(2004–2008) of 3-m pixel resolution hyperspectral
HyMap data of the Delta was acquired in late June of
each year and used to map and monitor the spread
of invasive species (Hestir et al. 2008; Khanna et
al. 2011b, 2015). AVIRIS-NG data acquisition was
funded through the California Department of Fish
and Wildlife (CDFW) to determine how drought
affected invasive species. It was flown in November
2014 and again in September 2015 (Ustin et al.
Up to 20 years of Landsat satellite data is available
with flyovers of California every 16 days. This
frequency of data acquisition, which is essential
for adaptive management, is a key advantage of
this relatively inexpensive remote sensing platform.
CDBW in cooperation with NASA Ames scientists
have field-validated that mapping the Delta has been
reliable. Landsat data has been used to study spatial
and temporal aspects of mechanical harvesting and
biocontrol. These data have also been applied to
studies of Delta management actions such as the
Emergency Drought Barrier in 2015, as well as to
mapping risk zones and providing insight on factors
that drive population spread. Landsat is currently the
only reliable platform able to track on a biweekly
basis the intra-annual (within-season) variation
in floating aquatic vegetation surface coverage,
climate effects, and the effects of integrated aquatic
weed management. Both Landsat and AVIRIS data,
contribute to analysis of inter-annual trends in
floating aquatic weed abundance and coverage. For
example, algorithms applied to Landsat data can
geographically isolate and pinpoint locations of
interest, such as persistent overwintering nursery
populations, for targeted early-season treatments.
These data can also track movement of water
hyacinth populations in the central and southern
Delta late in the field season. Knowledge of this
movement can direct control efforts and prevent
aquatic weeds in the south Delta from blocking fish
screens and pumps in the fall and winter, which is
critical to water conveyance in the state.
The floating aquatic vegetation community (FAV)
and the submerged community (SAV) can both
be mapped by a good multi-spectral sensor with
at least a few bands in the NearInfraRed (NIR) to
ShortWave InfraRed (SWIR) range of the electro-
magnetic spectrum and at least 5-×-5 meter spatial
resolution or better (Marshall and Lee 1994; Vis
et al. 2003; Phinn et al. 2008; Dogan et al. 2009;
Heblinski et al. 2011; Bresciani et al. 2012; Villa et
al. 2015). Sensors such as WorldView 2/3, IKONOS,
RapidEye, QuickBird, SPOT 5 through 7, etc. all
satisfy these criteria to some extent (Jensen 2000).
Landsat data with 30-×-30 meter pixels, though
free, is too coarse. Hyperspectral imagery, such as
used in this study, enables discrimination of FAV
genera as water hyacinth, water primrose, or the
native pennywort (Khanna et al. 2011b; Ustin et
al. 2015a, 2015c). Thus, it allows us to follow the
progress of each invasive plant individually, while
also keeping track of the native FAV community.
Efforts are currently underway to evaluate the
possibility of monitoring the Delta consistently
through remote sensing imagery. A group of multi-
agency scientists will develop a plan and evaluate the
ability of various sensors to map SAV and FAV, the
cost of acquisition, temporal repeatability, and other
requirements of a good monitoring program.
Continued monitoring of invasive aquatic
macrophytes in the Delta relies on the availability
of remote sensing data, either hyperspectral or
hyperspatial. In the past, the CDBW funded HyMap
image data collection from 2004 to 2008 and the
Mechanical Control
Mechanical control methods physically harvest
vegetation typically using large, power-driven
machinery. Although favored by some groups
because no herbicides are discharged into water,
several important drawbacks limit their use.
Fragmentation of many aquatic plants can create
propagules, which can float downstream and lead
to colonization of additional areas. Mechanically
shredding water hyacinth was found to increase
the overall abundance of carbon, nitrogen, and
phosphorus in the Delta water column, though
estuary-wide effects were limited (Greenfield et al.
2007). The Greenfield study provides an example of
a regional-scale assessment of how community-level
management actions can affect ecosystem processes.
More ecosystem-scale experiments and system
integration efforts in aquatic plant management are
In the Delta, limitations on using mechanical control
methods result primarily from concerns about fish
being killed during harvesting operations, accessing
sites with large machinery and/or gaining permits
and access to privately-owned but publically-
regulated levees to access the water with heavy
equipment, and the very high cost relative to
chemical control. Until 2016, mechanical control
could only be done in the winter (November to
March), and only on floating plants. Removal,
transport, and disposal of wet biomass is costly.
Despite this, the CDBW does now utilize mechanical
control methods both during the winter and in
critical locations during the chemical control
season. Because of concerns for listed fish species,
mechanical control is not permitted in certain areas,
or Delta-wide in the month of May.
To offset the cost and additionally provide a
beneficial reuse for harvested biomass, novel uses of
the plant material to produce bioenergy including
production of hydrogen or methane biofuels via
anaerobic digestion have been proposed (Wilkie
and Evans 2010). Pilot-scale projects are needed
to determine at what scale bioenergy production
is feasible using invasive aquatic plants harvested
from the Delta. A major concern associated with
programs such as these is that they could indirectly
CDFW funded AVIRIS–NG image data collection from
2014 to 2015. Although efforts to fund future short-
term data acquisition exist, the lack of a consistent
monitoring program severely limits invasive aquatic
macrophyte management.
Role of Citizen Science
Citizen science can be an effective tool for the early
detection, survey, and removal of invasive species,
though there has been little organized citizen science
activity in the Delta targeted at freshwater invasive
aquatic vegetation. Successful citizen science
models, such as the Invaders of Texas Program
and the Invasive Plant Atlas of New England, have
demonstrated that properly trained citizen scientists
are able to detect and report invasive plants in their
local areas and provide useful data to professional
scientists (Gallo and Waitt 2011). Actual removal
campaigns are likely to be only locally and
temporarily effective for widespread Delta invaders
like water hyacinth, Brazilian waterweed, and water
primrose, but may reduce or prevent the spread of
other relatively recent aquatic invasive species such
as South American spongeplant, or the introduction
of Hydrilla. The Smithsonian Environmental Research
Center (SERC) uses citizen scientists to help detect
invasive species such as invasive kelp (e.g., Undaria
pinnatifida) in their Kelp Watch program, which
targets the boating public and provides a website
for sighting reports. Several programs have been
developed that enable users to report locations and
take photographs of individual plants or populations
using smartphones, including iNaturalist and
EDDMapSWest. CalFlora’s Weed Manager includes a
suite of tools developed for organizations involved in
land management to track the locations of invasive
plants and treatments through time. These programs
are easy to use, do not require knowledge of GIS, and
allow for rapid reporting of invasive plant species.
These kinds of citizen science initiatives could be
adapted to detection and monitoring of freshwater
invasive aquatic vegetation in the Delta.
lead to conflicts for management priorities if energy
generation became dependent on the availability of
biomass from nuisance species.
Chemical Control
The chemical control of invasive aquatic plants
depends on the adaptive, strategic use of existing
permitted compounds, and, when available, their
integration with or replacement by newly-permitted
compounds. Issues that have arisen regarding
herbicide use, as observed in the state of Florida, are
herbicide resistance and non-target toxicity.
The following is a description of chemicals used for
aquatic vegetation control in the Delta. Diquat is an
example of a chemical that has been successfully
used to rapidly control Egeria. However, its toxicity
profile indicates some elevated risk to zooplankton
food chains. Fluridone, which is extensively used
in the Delta as well as in Florida, can be applied
in liquid, granular, and pellet forms. Pellets can
maintain a fluridone concentration of 3 ppb in the
water column for 12 weeks. The concentration must
be high enough to kill the target species but low
enough to be safe for non-target species. Penoxsulam
is a relatively new herbicide that Florida is utilizing
for floating plant management, but it is not currently
permitted for aquatic use in California. In the Delta,
2,4-D and glyphosate are two herbicides than have
been used to control water hyacinth. The DBW has
authorization to use a newer chemical, imazamox,
which may control water hyacinth (and water
primrose) more efficiently with reduced dosages than
the two older herbicides, which reduces control costs.
USDA–ARS is cooperating with the DBW to examine
the efficacy of imazamox, and possibly other
new herbicides, on water hyacinth and Brazilian
waterweed in comparison to 2,4-D and glyphosate.
The potential herbicide risks to aquatic species such
as a species of calanoid copepod (Eurytemora affinis)
and Delta Smelt (Hypomesus transpacificus) have
been investigated by Swee Teh’s research group at
UC Davis (DSC 2015). Teh’s research, funded by the
CDBW, examined the toxicity of fluridone, 2,4-D,
glyphosate, penoxsulam, and imazamox on E. affinis
and embryo and early larval stages of Delta Smelt.
Observed effects of herbicide toxicity on the test
species were found only at concentrations much
higher than those typically found in the water near
treated plants. The research group also found that
Delta Smelt are more sensitive to imazamox than are
E. affinis, although E. affinis was found to be more
sensitive to penoxsulam and the lipophillic herbicide
adjuvant, Agridex. Mixtures of penoxsulam and
Agridex were found to have additive effects on E.
affinis. Overall, based on these laboratory studies, the
effects of these herbicides at concentrations relevant
to their label-mandated use rates were non-detectable
to negligible on food webs and Delta Smelt.
Additional studies on Delta-specific food webs and
listed species are the focus of studies to be conducted
by the newly formed Aquatic Vegetation PWT and
are called for by the recently released Delta Smelt
Resiliency Strategy (CNRA 2016).
Biological Control
Biological control is a useful management tool for
invasive aquatic plants that are already widespread.
Biocontrol is defined as “the planned use of
undomesticated organisms (usually insects or plant
pathogens) to reduce the vigor, reproductive capacity,
or density of weeds.” (Cuda et al. 2008). Invasive
weeds, without the presence of their natural enemies,
are free to proliferate. Biocontrol introduces a natural
enemy, known as a biocontrol “agent,” to lower the
density of invasive species. The intentional release of
grass carp to control hydrilla, most typically in canals
and man-made lakes (Stocker and Hagstrom 1986),
is not germane to the Delta, because Hydrilla is not
present, and, in any event, grass carp are not host-
specific, precluding their use in Delta ecosystems.
Most typically, biological weed control involves
host-specific, non-native insects or mites from the
weed’s native range as natural enemies; native or
non-native pathogens have also occasionally been
considered. The process of implementing a biocontrol
management project involves the following:
1. Finding natural enemies through lab tests to
verify narrow (genus- or species-specific) host
specificity, determine the organism’s biological
life cycle, and demonstrate its efficacy (Briese
2005; Stastny et al. 2005; Suckling and Sforza
2. Providing extensive information for peer-review
and regulatory review processes at the federal,
state, and tribal level;
3. Receiving a permit for field release;
4. Completing additional permit processes to gain
site access at regional, state, and local levels;
5. Releasing the agent; and
6. Evaluating its establishment, dispersal, and effect.
Benefit-to-cost ratios range from 8:1 to 300:1
(Culliney 2005; Van Driesche et al. 2010; van Wilgen
et al. 2013). Research on and development of new
biological control agents, and performing initial
releases and impact evaluations, require up-front
costs ($3 to $8 million) and a substantial time
commitment (5 to 10 years). The release of agents,
once established, is irreversible. Biological control
typically takes 5 or more years for full effect and
does not eradicate the weed. However, effective
biological control agents will control invasive aquatic
plants in a very targeted manner, and control their
spread in the long-term through self-replenishment
and dispersal.
The status of biological agents for six major aquatic
weeds is summarized in Table 2. Two biocontrol
agents that target Arundo donax and three that target
water hyacinth have been released in the Delta; both
of these weeds are good targets because they don’t
have any close native relatives in California. To
date, only one of these five has been documented as
established in the Delta, the leaf-chewing and stem-
mining water hyacinth weevil, Neochetina bruchi;
its populations can be dense but its impact is not
sufficient to obviate other control methods (Akers et
al. 2017; Hopper et al. 2017). Some researchers are
actively seeking to obtain permission to re-release
another weevil, Neochetina eichhorniae, and to
release a planthopper, Megamelus scutellaris, that
feeds on vascular tissues, and that is established
outside of the Delta (Moran et al. 2016). The thermal
tolerance of the weevil is being studied by the
USDA–ARS in Albany (P. Moran, 2016a email to
J. Ta, unreferenced, see “Notes"), and the toxicity (or
lack thereof) of possible ingestion of the planthopper
and weevil by listed fish species is being investigated
by fish biologists at the University of California-
Davis in cooperation with USDA–ARS and the CDBW.
One candidate agent, a leaf-mining fly, that targets
Brazilian waterweed (Egeria densa) was rejected
because of non-target feeding in lab tests (P. Moran,
2016b email to J. Ta, unreferenced, see “Notes"). The
Ludwigia genus, which also has a native species,
Ludwigia palustris, is still in the genetic evaluation
process so biocontrol cannot yet be considered. The
USDA–ARS offices located in Albany and Davis,
California, have collaborative agreements with the
Fundación para el Estudio de Especies Invasivas
(FUEDEI) lab in Argentina, the native region
for invasive Ludwigia spp. and South American
spongeplant, so biocontrol agents may become
available in the future. Unlike the other invasive
aquatic weeds in the Delta, P. crispus has numerous
native relatives (same genus) in California, and it
might, therefore, be difficult to identify biocontrol
agents that are sufficiently host-specific.
Economic Incentives
The use of economic incentives to manage invasive
aquatic macrophytes has been a recent area of
development. For example, the expansion of fuel-cell
technology and its commercial applications, such
as hydrogen fuel cell-based fork lifts, may advance
expansion of commercial harvesting of invasive
aquatic plants for biofuel and fertilizer production.
Recent efforts have focused on harvesting water
hyacinth and possibly Arundo donax, which could
dramatically reduce plant cover. However, significant
economic hurdles have to be overcome, including
the cost to transport aquatic weed biomass from field
sites to processing sites, and the economies of scale
associated with constructing and operating these
facilities. The other limitation is the relatively short
season in the temperate environment of the Delta.
Though these are not intended to represent all
relevant recommendations on this topic, we
recommend the following, based on presentations
at the 2015 symposium, as well as our own expert
opinions as co-authors of this paper.
A consistent monitoring program for invasive
aquatic vegetation at several levels of accuracy
and precision is needed. The most widespread
Table 2 Status of biological control of six major aquatic weeds in the Delta
Species Introduced agents Effects and status Supporting studies
Water hyacinth
(Eichhornia crassipes)
Two weevils:
Neochetina bruchi
Neochetina eichhorniae
Introduced to the Delta along
with one moth (Niphograpta
albiguttalis) by USACE in
early 1980s.
• Only N. bruchi established in Delta (1985), and was later
detected (early 2000s to present). Currently widespread
and locally abundant both in and outside of the Delta, but
populations insufficient to reduce need for chemical and
mechanical control.
Stewart and
Cofrancesco (1988)
A South American delphacid
Megamelus scutellaris
Released in limited numbers
in 2011 by CDFA.
• Only confirmed establishment outside Delta, near Folsom,
California. Small initial release size, climate factors, or
pesticide drift may have affected Delta releases.
• Regulatory review (by USFWS and NOAA NMFS for CDBW-
Parks integrated adaptive management plan) needed for
widespread Delta releases.
Tipping et al. (2011)
Moran et al. (2016)
Giant reed
(Arundo donax)
An Arundo stem-galling
Tetramesa romana
Currently being released in
Delta by USDA ARS.
• Released in Texas, Arizona, and California.
T. romana only develops on genus Arundo with 90% of egg-
laying and feeding occurring at shoot tips, causing galls that
stunt plant growth.
• Observed in Texas to reduce Arundo stand biomass by 30%
to 40% after 7 years, fostering colonization by native plants
• Adventive in southern California (Ventura and Santa Ana
Goolsby et al. (2009)
Moran and Goolsby
Goolsby et al. (2014)
Moran et al. (2016)
Goolsby et al. (2016)
Moran et al. (2017)
An Arundo armored scale:
Rhizaspidiotus donacis
Released in Delta within last
two years.
• Due to recent release, no available data on impact of either
agent in the Delta region or elsewhere in California.
R. donacis observed in Texas to cause distortion and death of
lateral shoots and accumulations of scales on rhizomes.
Moran and Goolsby
Cortes et al. (2011a)
Cortes et al. (2011b)
Goolsby et al. (2011)
Brazilian waterweed
(Egeria densa)No agent introduced.
• USDA ARS initiated biological control studies in 2009.
• A fly species, Hydrellia egeriae, was found to feed and
reproduce on native Elodea canadensis during quarantine
tests and rejected.
• No other potential agents are in quarantine.
Water yellow-primrose
(Ludwigia hexapetala
or L. peploides ssp.
No agent introduced.
• Preliminary biocontrol studies began in 2010 in the native
range in South America.
• Critical questions being addressed by USDA ARS are genetic
identity of invasive California populations and polyploidy as
well as environmental factors that promote or limit growth.
• No agents are currently in quarantine.
Grewell et al. (2016)
South American
sponge plant
No agent introduced.
• No biological control project or pre-release risk assessment
has been initiated.
• CDFA successfully eradicated plant in Sonoma County and
reduced populations in Delta region as well as Shasta and
Merced counties. Management has transitioned to control
program under CDBW.
Curlyleaf pondweed
(Potamogeton crispus)No agent introduced. • No biological control project or pre-release risk assessment
has been initiated.
monitoring, using hyperspectral remote sensing
techniques and coordinated field campaigns, has
so far been funded through short-term grants,
limiting its potential longevity.
Robust funding is needed for effective early
detection and rapid response. The CDFA’s Hydrilla
Eradication Program exemplifies a very successful
early-detection and rapid-response strategy that
has prevented the introduction and establishment
of Hydrilla in the Delta and in hundreds of water
bodies in California. However, funding cuts in
recent years have weakened the effectiveness of
the program and may increase the possibility of
small, undetected populations of Hydrilla being
established in the Delta.
To prevent accumulation of large biomass,
policies need to enable early management of
invasive aquatic vegetation before their growth
accelerates. In the Delta, management actions
are often delayed until after exponential growth
occurs, making control much more challenging.
For example, herbicide treatments are not allowed
until March in the south Delta, and until mid-
summer in the north Delta.
Improved coordination among agencies is
necessary to achieve effective management.
To address this need, the USDA–ARS-funded
DRAAWP; the new Aquatic Vegetation PWT;
the DIISC are all bringing federal, state, regional,
and local agencies together, so expertise and new
funding opportunities can be leveraged. Efforts
like these should be sustained and expanded.
Research is needed to successfully develop fully
integrated control methods (chemical, mechanical,
and biological) and to manage the suite of
invasive aquatic plants—as opposed to single
species—at the population and community level.
Better coordination among agencies as mentioned
earlier would support this effort.
New options in chemical control depend on
continued research and development of new
active ingredients that may be used to decrease
the reliance on only one or two active ingredients.
Regulations restrict herbicide treatments both
spatially and temporally, and only a few
herbicides are currently approved. Herbicides with
new modes of action need to be approved so that
management agencies can rotate modes of action
to avoid resistance.
Biological control is a useful management tool
for controlling widespread invasive aquatic plants
that are beyond the point of eradication. Support
of and funding for the development of new
biocontrol agents should continue. In addition,
further studies are needed on how environmental
parameters such as water quality and plant
nutrient states influence biological control agents
and their establishment.
Synthesis of existing data and, possibly, new
studies are needed to evaluate the effects of
invasive aquatic vegetation and their control of
the habitat quality of listed fish species such as
Delta Smelt and salmonids. Many key regulations
that govern aquatic weed control in the Delta
result from statutes that require protection of
these species. In particular, the presumed risk
of early herbicide use on Endangered Species
Act (ESA)-listed fish populations needs to be
substantiated by toxicology studies, because
seasonally delayed management actions to protect
these fish result in massive invasive vegetation
biomass, exacerbating the management challenge.
A truly preventative program should be developed
and implemented. It should include inspections,
education, and training of marine managers,
boaters, anglers, and other key users of Delta
water bodies not only for early detection and
rapid response, but also as insurance to protect
management actions.
There are currently no major, organized citizen
science initiatives for detection, survey, and
removal of invasive aquatic vegetation in the
Delta, but other successful approaches could be
adapted for these purposes.
The September 15, 2015, symposium on invasive
aquatic vegetation at UC Davis, brought together
researchers and natural resource managers to discuss
the current knowledge and management of invasive
macrophytes in the Delta. Although there has been
ongoing foundational research on the environmental
drivers of aquatic plant growth, much remains
to be understood about the role invasive aquatic
vegetation play as ecosystem engineers in altering
water quality, nutrient levels, sedimentation, and
ecological communities, including effects on habitat
for listed fish species. With increased awareness
of macrophytes as ecosystem engineers, there is
a growing call to address management of aquatic
systems and community interactions at the ecosystem
level, not just to control individual plant species.
Although several options in mechanical, chemical,
and biological controls are available for management
of invasive macrophytes, notable challenges have
emerged that call for innovative solutions. In
particular, documented cases of herbicide resistance,
which have been observed in other states such as
Florida, call for management practices that aim to
minimize the development of herbicide resistance and
rotate herbicide modes of action.
Funding management actions via soft money allows for
prompt responses, but the lack of permanent funding
remains a barrier toward creation of stable long-term
monitoring and control programs (CDFG [now CDFW]
2008). For example, the hyperspectral remote sensing
data used to monitor the presence and distribution of
invasive aquatic vegetation described in this article
was funded for 2004–2008 and 2014–2015, and
recently funded for 2016, but future sustained funding
remains uncertain. In addition, programs critical for
rapid response and eradication such as the Hydrilla
Eradication Program have faced budget cuts in recent
Successful management of invasive aquatic
vegetation requires a clear identification of the
desired outcome, which may differ depending
on stakeholder values. In addition, the timing
of management action is important, and the
recommended management of high-priority
introduced species is through prevention, early
detection, rapid response, and eradication or control
(Anderson 2005; Williams and Grosholz 2008).
Approaching management at the population and
community level will help reduce the “squeaky
wheel” syndrome that often results in limited,
transient success which itself is thwarted by the
emergence of a formerly “minor” problem species.
Bioeconomic modeling also indicates that money is
better spent toward prevention of invasive species
establishment (Leung et al. 2002). Because of the
complexity of invasive macrophyte management,
integrated management efforts by disparate
organizations and agencies are critical. Although
successful ecosystem management of invasive
aquatic vegetation in the Delta remains an elusive
goal, ongoing development and implementation of
new strategies for adaptive, integrated management
of aquatic weeds is showing promise to achieve
improved management outcomes and enhance
protection of the Delta’s water resources.
The authors would like to extend thanks to the
Delta Science Program of the Delta Stewardship
Council and the Delta Interagency Invasive Species
Coordination Team for organizing the day-long
invasive aquatic weed symposium on September 15,
2015, and to U.C. Davis for hosting the event on
which this work is based. JHV was supported by the
UC Office of the President's Multi-Campus Research
Programs and Initiatives (MR-15-328473) through
UC Water, the University of California Water Security
and Sustainability Research Initiative.
Akers RP, Bergmann RW, Pitcairn MJ. 2017. Biological
control of water hyacinth in California’s Sacramento–
San Joaquin River Delta: observations on establishment
and spread. Biocontrol Sci Tech 27(6):755-768.
Anderson LWJ. 2005. California’s reaction to Caulerpa
taxifolia: a model for invasive species rapid response.
Biol Inv 7(6):1003-1016.
Barnes MA, Jerde CL, Wittmann ME, Chadderton WL,
Ding J, Zhang J, Purcell M, Budhathoki M, Lodge DM.
2014. Geographic selection bias of occurrence data
influences transferability of invasive Hydrilla verticillata
distribution models. Ecol Evol 4(12):2584-2593.
Boyer K, Sutula M. 2015. Factors controlling submersed
and floating macrophytes in the Sacramento–
San Joaquin Delta. Prepared for Central Valley
Regional Water Quality Control Board and California
Environmental Protection Agency State Water Resources
Control Board.
Bresciani M, Bolpagni R, Braga F, Oggioni A, Giardino C.
2012. Retrospective assessment of macrophytic
communities in southern Lake Garda (Italy) from in situ
and mivis (multispectral infrared and visible imaging
spectrometer) data. J Limnol 71(1).
Briese DT. 2005. Translating host-specifcity test results into
the real world: The need to harmonize the yin and yang
of current testing procedures. Biol Control 35:208-214.
Borgnis E, Boyer KE. 2015. Salinity tolerance and
competition drive distributions of native and invasive
submerged aquatic vegetation in the upper San Francisco
Estuary. Estuaries Coasts
[CDFA] California Department of Food and Agriculture.
[date unknown]. Aquatic pesticide application plan
for the California Department of Food and Agriculture
Hydrilla eradication program. [cited 12 Dec 2017].
Prepared for the State Water Resources Control Board.
Available from:
[CDFA] California Department of Food and Agriculture.
[date unknown]. Food and Agriculture Code, Section
7271. [cited 20 December 2017]. Available from: http://
[CDFG] California Department of Fish and Game. 2008.
California aquatic invasive species management plan.
Sacramento, CA: CDFG.
[CNRA] California Natural Resources Agency. 2016. Delta
Smelt resiliency strategy. Sacramento (CA): CNRA. p. 11.
Cohen AN, Carlton JT. 1998. Accelerating invasion rate in
a highly invaded estuary. Science 279.
Cook CDK, Urmi-Konig K. 1984. A revision of the genus
Egeria (Hydrocharitaceae). Aquat Bot 19:73-96.
Cortes E, Goolsby JA, Moran PJ, Marcos-Garcia MA.
2011a. The effect of the armored scale, Rhizaspidiotus
donacis (Hemiptera: Diaspididae), on shoot growth of the
invasive plant Arundo donax (Poaceae: Arundinoideae).
Biocontrol Sci Technol 21(5):535-545.
Cortes E, Kirk AA, Goolsby JA, Moran PJ, Racelis AE,
Marcos-Garcia MA. 2011b. Impact of the Arundo scale
Rhizaspidiotus donacis (Hemiptera: Diaspididae) on
the weight of Arundo donax (Poaceae: Arundinoideae)
rhizomes in Languedoc southern France and
mediterranean Spain. Biocontrol Sci Technol
Cuda JP, Charudattan R, Grodowitz MJ, Newman RM,
Shearer JF, Tamayo ML, Villegas B. 2008. Recent
advances in biological control of submersed aquatic
weeds. J Aquat Plant Manage 46:15-32. Available from:
Culliney TW. 2005. Benefits of classical biological control
for managing invasive plants.
Critical Rev Plant Sci 24(2):131-150.
Dahm CN, Parker AE, Adelson AE, Christman, MA,
Bergamaschi BA. 2016. Nutrient dynamics of the
Delta: effects on primary producers. San Franc Estuary
Watershed Sci [Internet]. [cited 12 Nov 2017];14:4(4).
Dogan OK, Akyurek Z, Beklioglu M. 2009. Identification
and mapping of submerged plants in a shallow lake
using Quickbird satellite data.
J Environ Manage. 90:2138-2143.
[DSC] Delta Stewardship Council. 2015. September 15
- science symposium on invasive aquatic vegetation.
Sacramento, CA; [accessed 2017 August 15, 2017].
[FDFWCC] Florida Department of Fish and Wildlife
Conservation Commission. 2012. Program criteria and
standards, Final 68F-54.001. In: Florida Administrative
Code and Administrative Register [Internet]. [cited
12 Dec 2017]. Availlable from:
Finlayson BJ. 1983. Water hyacinth: threat to the Delta?
Outdoor California. Sacramento (CA): CDFW. p. 10-14.
Gallo T, Waitt D. 2011. Creating a successful citizen
science model to detect and report invasive species.
Bioscience 61(6):459-465.
Goolsby JA, Gaskin JF, Tarin DV, Pepper AE, Henne DC,
Auclair A, Racelis AE, Summy KR, Moran PJ,
Thomas DB, et al. 2014. Establishment and spread of
a single parthenogenic genotype of the Mediterranean
Arundo wasp, Tetramesa romana, in the variable climate
of Texas. Southw Entomol 39(4):675-690.
Goolsby JA, Kirk AA, Moran PJ, Racelis AE, Adamczyk JJ,
Cortés E, García MÁM, Jimenez MM, Summy KR,
Ciomperlik MA, et al. 2011. Establishment of the
armored scale, Rhizaspidiotus donacis, a biological
control agent of Arundo donax. Southw Entomol
Goolsby JA, Moran PJ, Racelis AE, Summy KR,
Jimenez MM, Lacewell RD, Perez de Leon A, Kirk AA.
2016. Impact of the biological control agent Tetramesa
romana (hymenoptera: Eurytomidae) on Arundo donax
(Poaceae: Arundinoideae) along the Rio Grande River in
Texas. Biocontrol Sci Technol 26(1):47-60.
Goolsby JA, Spencer D, Whitehand L. 2009. Pre-
release assessment of impact on Arundo donax by the
candidate biological control agents Tetramesa romana
(Hymenoptera: Eurytomidae) and Rhizaspidiotus donacis
(Hemiptera: Diaspididae) under quarantine conditions.
Southw Entomol 34(4):359-376.
Gordon DR, Gantz CA, Jerde CL, Chadderton WL,
Keller RP, Champion PD. 2012. Weed risk assessment for
aquatic plants: modification of a New Zealand system
for the United States. PloS One 7(7):e40031.
Green JC. 2005. Modelling flow resistance in vegetated
streams: review and development of new theory.
Hydrol Process 19:1245-1259.
Greenfield BK, Siemering GS, Andrews JC, Rajan M,
Andrews SP, Spencer DF. 2007. Mechanical shredding of
water hyacinth (Eichhornia crassipes): Effects on water
quality in the Sacramento–San Joaquin River Delta,
California. Estuaries Coasts 30(4):627-640.
Grewell BJ, Netherland MD, Thomason MJS. 2016.
Establishing research and management priorities for
invasive water primrose (Ludwigia spp.). U.S. Army
Corps of Engineers.
Haller WT, Sutton DL, Barlowe WC. 1974. Effects of
salinity on growth of several aquatic macrophytes.
Ecology 55(4):891-894.
Heblinski J, Schmieder K, Heege T, Agyemang TK,
Sayadyan H, Vardanyan L. 2011. High-resolution
satellite remote sensing of littoral vegetation of
Lake Sevan (Armenia) as a basis for monitoring and
assessment. Hydrobiologia 661:97-111.
Hestir EL, Khanna S, Andrew ME, Santos MJ, Viers JH,
Greenberg JA, Rajapakse SS, Ustin SL. 2008.
Identification of invasive vegetation using hyperspectral
remote sensing in the California Delta ecosystem.
Remote Sens Environ 112(11):4034-4047.
Hopper JV, Pratt PD, McCue KF, Pitcairn MJ, Moran PJ,
Madsen JD. 2017. Spatial and temporal variation of
biological control agents associated with Eichhornia
crassipes in the Sacramento–San Joaquin River Delta,
California. Biol Control 111(1):13-22.
Jensen JR. 2000. Remote sensing of the environment: an
earth resource perspective. Upper Saddle River (NJ):
Prentice Hall.
Khanna S, Santos MJ, Hestir EL, Ustin SL. 2011a.
Plant community dynamics relative to the changing
distribution of a highly invasive species, Eichhornia
crassipes: a remote sensing perspective.
Biol Inv 14(3):717-733.
Khanna S, Santos MJ, Ustin SL, Haverkamp PJ. 2011b.
An integrated approach to a biophysiologically based
classification of floating aquatic macrophytes.
Int J Remote Sens 32(4):1067-1094.
Leung B, Lodge DM, Finnoff D, Shogren JF, Lewis MA,
Lamberti G. 2002. An ounce of prevention or a pound of
cure: Bioeconomic risk analysis of invasive species.
Proc Roy Sci B 269(1508):2407-2413.
Light T, Grosholz T, Moyle P. 2005. Delta ecological
survey (Phase 1): nonindigenous aquatic species in the
Sacramento–San Joaquin Delta, a literature review.
Stockton (CA): Report submitted to U.S. Fish and
Wildlife Service.
Marshall TR, Lee PF. 1994. Mapping aquatic macrophytes
through digital image analysis of aerial photographs: As
assessment. J Aquat Plant Manage 32:61-66.
Moran PJ, Goolsby JA. 2009. Biology of the galling wasp
tetramesa romana, a biological control agent of giant
reed. Biol Control 49:169-179.
Moran PJ, Goolsby JA. 2010. Biology of the armored
scale Rhizaspidiotus donacis (Hemiptera: Diaspididae), a
candidate agent for biological control of giant reed. Ann
Entomol Soc Am 103(2):252-263.
Moran PJ, Pitcairn MJ, Villegas B. 2016. First
establishment of the planthopper, Megamelus scutellaris
Berg, 1883 (Hemiptera: Delphacidae), released for
biological control of water hyacinth in California.
Pan-Pacific Entomol 92(1):32-43.
Moran PJ, Vacek AT, Racelis AE, Pratt PD, Goolsby JA.
2017. Impact of the Arundo wasp, Tetramesa romana
(Hymenoptera: Eurytomidae) on biomass of the invasive
weed, Arundo donax (Poaceae: Arundinoideae) and on
revegetation of riparian habitat along the rio grande in
texas. Biocontrol Sci Technol 27(1):96-114.
Okada M, Grewell BJ, Jasieniuk M. 2009. Clonal spread
of invasive Ludwigia hexapetala and L. grandiflora in
freshwater wetlands of California.
Aquat Bot 91(3):123-129.
Penfound WT, Earle TT. 1948. The biology of the water
hyacinth. Ecol Monogr 18(4):447-472.
Petticrew EL, Kalff J. 1992. Water flow and clay retention
in submerged macrophyte beds. Can J Fish Aquat Sci
Phinn S, Roelfsema C, Dekker A, Brando V, Anstee J. 2008.
Mapping seagrass species, cover and biomass in shallow
waters: An assessment of satellite multi-spectral and
airborne hyper-spectral imaging systems in Moreton
Bay (Australia). Remote Sens Environ 112:3413-3425.
[SFEP] San Francisco Estuary Partnership. 2015. The state
of the estuary 2015: status and trends updates on 33
indicators of ecosystem health. Oakland (CA): SFEP.
Shih SF, Rahi GS. 1981. Seasonal variations of manning’s
roughness coefficient in a subtropical marsh. Trans Am
Soc Agric Eng 25(1):116-119.
Stastny M, Schaffner U, Elle E. 2005. Do vigour of
introduced populations and escape from specialist
herbivores contribute to invasiveness? J Ecol 93:27-37.
Stewart RM, Cofrancesco AF. 1988. Biological control of
water hyacinth in the California Delta. Sacramento (CA):
U.S. Army Corps of Engineers.
Stocker RK, Hagstrom NT. 1986. Control of submerged
aquatic plants with triploid grass carp in southern
California irrigation canals.
Lake Reservoir Manage 2(1):41-45.
Suckling DM, Sforza RFH. 2014. What magnitude are
observed non-target impacts from weed biocontrol?
PloS One 9(1).
Téllez TR, López E, Granado G, Pérez E, López R,
Guzmán J. 2008. The water hyacinth, Eichhornia
crassipes: an invasive plant in the Guadiana River basin
(Spain). Aquat Inv 3(1):42-53.
Tipping PW, Center TD, Sosa AJ, Draw FA. 2011.
Host specificity assessment and potential impact of
Megamelus scutellaris (Hemiptera: Delphacidae) on
water hyacinth Eichhornia crassipes (Pontederiales:
Pontederiaceae). Biocontrol Sci Technol 21(75-87).
Toft JD, Simenstad CA, Cordell JR, Grimaldo LF. 2003. The
effects of introduced water hyacinth on habitat structure,
invertebrate assemblages, and fish diets. Estuaries
Ustin SL, Khanna S, Bellvert J, Shapiro K. 2015a. Impact
of drought on submerged aquatic vegetation (SAV)
and floating aquatic vegetation (FAV) using AVIRIS–
NG airborne imagery. Report submitted to California
Department of Fish and Wildlife. Available upon request
Ustin SL, Khanna S, Bellvert J, Shapiro K. 2015b. Impact
of drought on submerged aquatic vegetation (SAV)
and floating aquatic vegetation (FAV) using AVIRIS–
NG airborne imagery. Report submitted to California
Department of Fish and Wildlife. Available upon request
Ustin SL, Santos MJ, Hestir EL, Khanna S, Casas A,
Greenberg J. 2015c. Developing the capacity to monitor
climate change impacts in Mediterranean estuaries. Evol
Ecol Res 16(6):529-550. Available from: http://www.
Van Driesche RG, Carruthers RI, Center T, Hoddle MS,
Hough-Goldstein J, Morin L, Smith L, Wagner DL,
Blossey B, Brancatini V, et al. 2010. Classical biological
control for the protection of natural ecosystems.
Biol Control 54:S2-S33.
Van TK, Steward KK. 1990. Longevity of monoecious
Hydrilla propagules. J Aquat Plant Manage 28:74-76.
van Wilgen BW, Moran VC, Hoffmann JH. 2013. Some
perspectives on the risks and benefits of biological
control of invasive alien plants in the management of
natural ecosystems. Environ Manage 52(3):531-540.
Villa P, Bresciani M, Bolpagni R, Pinardi M, Giardino C.
2015. A rule-based approach for mapping macrophyte
communities using multi-temporal aquatic vegetation
indices. Remote Sens Environ 171:218-233. http://www.
Villamagna AM, Murphy BR. 2010. Ecological and socio-
economic impacts of invasive water hyacinth (Eichhornia
crassipes): A review. Freshw Biol 55(2):282-298.
Vis C, Hudon C, Carignan R. 2003. An evaluation
of approaches used to determine the distribution
and biomass of emergent and submerged aquatic
macrophytes over large spatial scales. Aquat Bot 77:187-
Whipple AA, Grossinger RM, Rankin D, Stanford B,
Askevold RA. 2012. Sacramento–San Joaquin Delta
historical ecology investigation: exploring pattern and
process. Prepared for the California Department of
Fish and Wildlife and Ecosystem Restoration Program.
A report of SFEI–ASC's historical ecology program.
Richmond (CA): San Francisco Estuary Institute–Aquatic
Science Center.
Wilkie AC, Evans JM. 2010. Aquatic plants: an opportunity
feedstock in the age of bioenergy. Biofuels 1(2):311-321.
Williams SL, Grosholz ED. 2008. The invasive species
challenge in estuarine and coastal environments:
marrying management and science.
Estuaries Coasts 31(1):3-20.
Foschi P, Liu H. 2002. Active learning for classifying
a spectrally variable subject. Paper presented at: 2nd
International Workshop on Pattern Recognition for
Remote Sensing; 16 Aug 2002; Niagara Falls, Canada.
Khanna S, Bellvert J, Shapiro K, Ustin SL. 2015. Dynamic
changes in an estuary over the past 11 years. Paper
presented at: American Geophysical Union Fall Meeting;
14-18 Dec 2017; San Francisco, CA, USA.
Moran P. 2016a. Email communication to Jenny Ta
regarding thermal tolerance testing by the USDA–ARS
EIWRU of accessions of the water hyacinth weevil
Neochetina eichhorniae.
Moran P. 2016b. Email communication to Jenny Ta
regarding host range testing by the USDA–ARS EIWRU
of the Egeria fly Hydrellia spp.
... Slow-moving warm-water conditions expanded, favoring non-native aquatic species, including plants. Aquatic weed management could not stop the steady expansion of Egeria densa and other submersed aquatic macrophytes throughout the Delta (Ta et al. 2017;Hard 2018). The expansion of submersed aquatic vegetation likely aided Largemouth Bass and sunfish populations, which use it for habitat (Brown 2003;Nobriga and Feyrer 2007;Ferrari et al. 2014). ...
... Many uncertainties remain, especially given the high temperatures and low snowpack that distinguished this drought from previous recorded droughts. The extreme conditions created new uncertainties around the long-term effects of brackish irrigation water on Delta soil conditions (Aegerter and Leinfelder-Miles 2016); maintaining the Shasta cold-water pool to support winterrun Chinook Salmon (Mount et al. 2017b); the effect of hatchery supplementation on winterrun Chinook Salmon (NMFS 2016); the effects of increased straying of fall-run Chinook Salmon from trucking (Dedrick and Baskett 2018); the control of long-term expansion of some aquatic weeds (Ta et al. 2017); and the response of Microcystis to drought and water quality . ...
Full-text available
This paper reviews environmental management and the use of science in the Sacramento–San Joaquin Delta during California’s 2012–2016 drought. The review is based on available reports and data, and guided by discussions with 27 agency staff, stake-holders, and researchers. Key management actions for the drought are discussed relative to four major drought water management priorities stated by water managers: support public health and safety, control saltwater intrusion, preserve cold water in Shasta Reservoir, and maintain minimum protections for endangered species. Despite some success in streamlining communication through interagency task forces, conflicting management mandates sometimes led to confusion about priorities and actions during the drought (i.e., water delivery, the environment, etc.). This report highlights several lessons and offers suggestions to improve management for future droughts. Recommendations include use of pre-drought warnings, timely drought declarations, improved transparency and useful documentation, better scientific preparation, development of a Delta drought management plan (including preparing for salinity barriers), and improved water accounting. Finally, better environmental outcomes occur when resources are applied to improving habitat and bolstering populations of native species during inter-drought periods, well before stressful conditions occur.
... While pelagic productivity has declined in the estuary's bays and channels over the years (Kimmerer et al. 1994;Mac Nally et al. 2010;Thomson et al. 2010), productivity in the shallow littoral areas (e.g., near-shore habitats such as levee margins) within the Delta appears to have increased. Submerged and floating aquatic vegetation have become more widely distributed over the past few decades, and the abundances of non-native fishes associated with this vegetations have also increased (Brown and Michniuk 2007;Conrad et al. 2016;Mahardja et al. 2017;Ta et al. 2017). ...
Full-text available
Climate change is intensifying the effects of multiple interacting stressors on aquatic ecosystems worldwide. In the San Francisco Estuary, signals of climate change are apparent in the long-term monitoring record. Here we synthesize current and potential future climate change effects on three main ecosystems (floodplain, tidal marsh, and open water) in the upper estuary and two representative native fishes that commonly occur in these ecosystems (anadromous Chinook Salmon, Oncorhynchus tshawytscha and estuarine resident Sacramento Splittail, Pogonichthys macrolepidotus). Based on our review, we found that the estuary is experiencing shifting baseline environmental conditions, amplification of extremes, and restructuring of physical habitats and biological communities. We present priority topics for research and monitoring, and a conceptual model of how the estuary currently functions in relation to climate variables. In addition, we discuss four tools for management of climate change effects: regulatory, water infrastructure, habitat development, and biological measures. We conclude that adapting to climate change requires fundamental changes in management.
... It was first reported in California in Yolo County in 1904 (Bock 1968) and has become a widespread invasive species in the Sacramento-San Joaquin River Delta (hereafter the Delta). It is estimated to cover approximately 1,214 ha (3,000 ac) in the Delta (Ta et al. 2017). ...
Full-text available
The California State Parks Division of Boating and Waterways (CDBW) manages waterhyacinth [Eichhornia crassipes (Mart.) Solms] in the Sacramento-San Joaquin River Delta (the Delta) to ensure navigation and fish habitat. Decreasing the amount of waterhyacinth in the Delta should increase the proportion of oxygenated water for fish habitat and migration. However, some water resource management personnel are concerned that plant decomposition following herbicide treatment could temporarily lower the dissolved oxygen in the water under the plant canopy. The U.S. Department of Agriculture Agricultural Research Service and CDBW conducted an experiment in the summer of 2016 to monitor dissolved oxygen following herbicide treatment relative to untreated waterhyacinth canopies and open water. The experiment was conducted in two Delta environments. The first consisted of channels subject to tidal fluctuations and mass flow, and the second consisted of back-end sloughs, which have only a single outlet, where there was minimal water movement. Three channel-side sites were chosen and three 0.025-ha plots per site were randomly assigned to be treated with glyphosate or 2,4-D, or remain untreated. Three back end slough sites were chosen and three 0.25-ha plots per site were randomly assigned to be treated with imazamox, glyphosate, or 2,4-D, or remain untreated. A data logger measuring dissolved oxygen every 30 min was deployed under the canopy in each plot. Data was collected 2 wk prior to treatment through 6 wk after treatment. Data loggers were also deployed in the open water, away from waterhyacinth canopy, in the channels and in the back-end sloughs. The dissolved oxygen under waterhyacinth was consistently lower than in the open water. The dissolved oxygen levels pre-and posttreat-ment were compared for each treatment, using ANOVA (P 0.05). For the channel-side trial, there was no significant difference in dissolved oxygen levels for any of the treatments (P. 0.18). In the back-end sloughs, there was no significant difference in dissolved oxygen levels for any of the treatments (P. 0.92 for all comparisons). Herbicide treatments did not result in a significant decline in dissolved oxygen after treatment relative to pretreatment levels.
... In addition, all of these wetland sites will experience at least some growth of Azolla spp. (Valach et al., 2021; especially Azolla filiculoides), which is a native species (Ta et al., 2017) in the region and can fix large amounts of atmospheric N through their symbiotic relationship with the cyanobacterium Anabaena azolae (Carrapiço, 2010), providing additional N input to the system. Based on vegetation development, peat accumulation and soil C/N ratio we consider Mayberry and West Pond as mature wetlands and Sherman Wetland together with East End as young wetlands. ...
The concentration of nitrous oxide (N2O), an ozone-depleting greenhouse gas, is rapidly increasing in the atmosphere. Most atmospheric N2O originates in terrestrial ecosystems, of which the majority can be attributed to microbial cycling of nitrogen in agricultural soils. Here, we demonstrate how the abundance of nitrogen cycling genes vary across intensively managed agricultural fields and adjacent restored wetlands in the Sacramento-San Joaquin Delta in California, USA. We found that the abundances of nirS and nirK genes were highest at the intensively managed organic-rich cornfield and significantly outnumber any other gene abundances, suggesting very high N2O production potential. The quantity of nitrogen transforming genes, particularly those responsible for denitrification, nitrification and DNRA, were highest in the agricultural sites, whereas nitrogen fixation and ANAMMOX was strongly associated with the wetland sites. Although the abundance of nosZ genes was also high at the agricultural sites, the ratio of nosZ genes to nir genes was significantly higher in wetland sites indicating that these sites could act as a sink of N2O. These findings suggest that wetland restoration could be a promising natural climate solution not only for carbon sequestration but also for reduced N2O emissions.
... Phenylobacterium can obtain its energy for growth from xenobiotic compounds including those associated with herbicides (Eberspächer and Lingens, 2006). Herbicides are applied regularly in USFE to control aquatic weeds and can enter rivers through urban and agricultural runoff (Ta et al., 2017). The potential influence of herbicides on cyanobacteria and green algae growth in USFE was supported by a recent bioassay study in which the mortality of Microcystis and Chlamydomonas was less compared with the diatom Thalassiosira when exposed to Fluridone, a common herbicide used in USFE (Lam et al., 2019). ...
Full-text available
Microcystis blooms have occurred in upper San Francisco Estuary (USFE) since 1999, but their potential impacts on plankton communities have not been fully quantified. Five years of field data collected from stations across the freshwater reaches of the estuary were used to identify the plankton communities that covaried with Microcystis blooms, including non-photosynthetic bacteria, cyanobacteria, phytoplankton, zooplankton, and benthic genera using a suite of analyses, including microscopy, quantitative PCR (qPCR), and shotgun metagenomic analysis. Coherence between the abundance of Microcystis and members of the plankton community was determined by hierarchal cluster analysis (CLUSTER) and type 3 similarity profile analysis (SIMPROF), as well as correlation analysis. Microcystis abundance varied with many cyanobacteria and phytoplankton genera and was most closely correlated with the non-toxic cyanobacterium Merismopoedia , the green algae Monoraphidium and Chlamydomonas , and the potentially toxic cyanobacteria Pseudoanabaena , Dolichospermum , Planktothrix , Sphaerospermopsis , and Aphanizomenon . Among non-photosynthetic bacteria, the xenobiotic bacterium Phenylobacterium was the most closely correlated with Microcystis abundance. The coherence of DNA sequences for phyla across trophic levels in the plankton community also demonstrated the decrease in large zooplankton and increase in small zooplankton during blooms. The breadth of correlations between Microcystis and plankton across trophic levels suggests Microcystis influences ecosystem production through bottom-up control during blooms. Importantly, the abundance of Microcystis and other members of the plankton community varied with wet and dry conditions, indicating climate was a significant driver of trophic structure during blooms.
... Il existe plusieurs méthodes d'éradication des plantes invasives, mais le plus souvent ces méthodes ne permettent que d'atténuer leurs impacts en les contrôlant (DiTomaso, 2000). On peut citer par exemple, la lutte biologique 5 et l'utilisation de produits chimiques (Ta et al., 2017). Le gastéropode Lymnaea stagnalis peut être utilisé comme lutte biologique, car il consomme les élodées notamment Elodea nuttallii, faisant ainsi diminuer son expansion, ils ne sont en revanche pas d'une efficacité suffisante, mais peuvent être utilisés en complément d'une autre méthode de lutte (Boiché et al., 2011). ...
Dans un contexte d’expansion des espèces invasives, leur survie et succès sont conditionnés par leur capacité à s’adapter. En France, Ludwigia grandiflora (jussie) a envahi bon nombre de biotopes aquatiques et son déploiement récent dans les prairies humides a conduit à l’apparition de deux morphotypes, l’un aquatique et l’autre dit « terrestre ». L’objectif de cette thèse visait à mieux comprendre les capacités d’acclimatation de la jussie au milieu terrestre en explorant les sources de flexibilité que sont les mécanismes génétiques et épigénétiques. Les réponses des morphotypes aquatique et terrestre à différentes contraintes hydriques ont été évaluées via l’observation des traits morphologiques et développementaux, des dosages de métabolites et de phytohormones. La piste épigénétique a été abordée par l’utilisation d’une drogue hypométhylante, la zébularine. Ces travaux ont montré que L. grandiflora adapte son développement et son métabolisme avec des valeurs de biomasses élevées etLe morphotype terrestre présente des valeurs de traits plus importants que ceux du morphotype aquatique, quelle que soit la condition. Cependant, la plasticité phénotypique est plus importante chez le morphotype aquatique. Enfin, l’épigénétique via la méthylation de l’ADN semble impliquée dans la transition du morphotype aquatique vers le milieu terrestre. Nos résultats suggèrent une implication de la méthylation de l’ADN et de la plasticité phénotypique dans la réponse de la jussie au changement de milieu. Le morphotype terrestre ayant des capacités supérieures au morphotype aquatique, sa
... 'Omic approaches, applied across multiple phytoplankton species, can therefore help algal bloom occurrences, species distribution, and habitat requirements -as well as the impact of contaminants -to be better understood; for example, herbicides that are directly applied to surface waters to control invasive aquatic vegetation such as Brazilian waterweed (Egeria densa), water hyacinth (Eichhornia crassipes), and water primrose (Ludwigia spp.) ( Ta et al. 2017) on primary production. Conducting mechanistic studies across trophic levels is of particular importance in a system that has been described as food limited (Kimmerer et al. 2018), because this will provide information needed to address the cause, rather than treating the symptom. ...
Full-text available
Legacy and current-use contaminants enter into and accumulate throughout the San Francisco Bay−Delta (Bay−Delta), and are present at concentrations with known effects on species important to this diverse watershed. There remains major uncertainty and a lack of focused research able to address and provide understanding of effects across multiple biological scales, despite previous and ongoing emphasis on the need for it. These needs are challenging specifically because of the established regulatory programs that often monitor on a chemical-by-chemical basis, or in which decisions are grounded in lethality-based endpoints. To best address issues of contaminants in the Bay−Delta, monitoring efforts should consider effects of environmentally relevant mixtures and sub-lethal impacts that can affect ecosystem health. These efforts need to consider the complex environment in the Bay−Delta including variable abiotic (e.g., temperature, salinity) and biotic (e.g., pathogens) factors. This calls for controlled and focused research, and the development of a multi-disciplinary contaminant monitoring and assessment program that provides information across biological scales. Information gained in this manner will contribute toward evaluating parameters that could alleviate ecologically detrimental outcomes. This review is a result of a Special Symposium convened at the University of California−Davis (UCD) on January 31, 2017 to address critical information needed on how contaminants affect the Bay−Delta. The UCD Symposium focused on new tools and approaches for assessing multiple stressor effects to freshwater and estuarine systems. Our approach is similar to the recently proposed framework laid out by the U.S. Environmental Protection Agency (USEPA) that uses weight of evidence to scale toxicological responses to chemical contaminants in a laboratory, and to guide the conservation of priority species and habitats. As such, we also aimed to recommend multiple endpoints that could be used to promote a multi-disciplinary understanding of contaminant risks in Bay−Delta while supporting management needs.
Estuaries are ecologically and economically important ecosystems but are threatened by non-native invasive species, including many species of submersed aquatic vegetation (SAV). Herbicides are the primary tool used to control SAV, but most field evaluations of herbicides have been conducted in lentic systems. Therefore, managers working in estuaries must base their SAV control programs largely on findings from systems fundamentally different from their own. We conducted a study in the Sacramento-San Joaquin Delta to determine efficacy of the widely used herbicide fluridone in an estuarine ecosystem. The primary goal of SAV removal was restoration of open water habitat for endangered Hypomesus transpacificus (Delta Smelt). Over 18 months and multiple sets of multi-week fluridone applications, we monitored concentrations of fluridone and responses by SAV across pairs of treated and reference sites. Fluridone concentrations in the water were generally below the 2–5 parts per billion required for SAV control. Monitoring demonstrated that these low water concentrations were likely due to dissipation by tides, despite use of pelleted fluridone formulations marketed for flowing water environments. Fluridone did, however, accumulate in sediment at concentrations hundreds of times higher than those measured in the water. Nonetheless, we did not observe lasting reductions in SAV abundance or changes in SAV community composition. By demonstrating lack of efficacy of one of the few herbicides permitted for use in this estuary, this study highlights the need for development of SAV management tools tailored to the challenges of hydrologically complex environments like estuaries.
Full-text available
The Sacramento-San Joaquin Delta has been invaded by several species of non-native predatory fish that are presumed to be impeding native fish population recovery efforts. Since eradication of predators is unlikely, there is substantial interest in removing or altering manmade structures in the Delta that may exacerbate predation on native fish (contact points). It is presumed that these physical structures influence predator-prey dynamics, but how habitat features influence species interactions is poorly understood, and physical structures in the Delta that could be remediated to benefit native fish have not been inventoried completely. To inform future research efforts, we reviewed literature that focused on determining the effects of predator-prey interactions between fish, based on contact points that are commonly found in the Delta. We also performed a geospatial analysis to determine the extent of potential contact points in the Delta. We found SFEWS Volume 17 | Issue 4 | Article 1 that the effects of submerged aquatic vegetation (SAV) and artificial illumination are well studied and documented to influence predation in other freshwater systems worldwide. Conversely, other common structures in the Delta-such as docks, pilings, woody debris, revetment, and water diversions-did not have the same breadth of research. In the Delta, the spatial extent of the different types of contact points differed considerably. For example, 22% of the Delta water surface area is occupied by SAV, whereas docks only cover 0.44%. Our conclusion, based on both the literature review and spatial analysis, is that the effects of SAV and artificial illumination on predation warrant the most immediate future investigation in the Delta.
Full-text available
Increasing clarity of Delta waters, the emergence of harmful algal blooms, the proliferation of aquatic water weeds, and the altered food web of the Delta have brought nutrient dynamics to the forefront. This paper focuses on the sources of nutrients, the transformation and uptake of nutrients, and the links of nutrients to primary producers. The largest loads of nutrients to the Delta come from the Sacramento River with the San Joaquin River seasonally important, especially in the summer. Nutrient concentrations reflect riverine inputs in winter and internal biological processes during periods of lower flow with internal nitrogen losses within the Delta estimated at approximately 30% annually. Light regime, grazing pressure, and nutrient availability influence rates of primary production at different times and locations within the Delta. The roles of the chemical form of dissolved inorganic nitrogen in growth rates of primary producers in the Delta and the structure of the open-water algal community are currently topics of much interest and considerable debate. Harmful algal blooms have been noted since the late 1990s, and the extent of invasive aquatic macrophytes (both submerged and free-floating forms) has increased especially during years of drought. Elevated nutrient loads must be considered in terms of their ability to support this excess biomass. Modern sensor technology and networks are now deployed that make high-frequency measurements of nitrate, ammonium, and phosphate. Data from such instruments allow a much more detailed assessment of the spatial and temporal dynamics of nutrients. Four fruitful directions for future research include utilizing continuous sensor data to estimate rates of primary production and ecosystem respiration, linking hydrodynamic models of the Delta with the transport and fate of dissolved nutrients, studying nutrient dynamics in various habitat types, and exploring the use of stable isotopes to trace the movement and fate of effluent-derived nutrients.
Technical Report
Full-text available
Creeping water primroses and water primrose-willows are among the most aggressive aquatic invasive plant invaders in the world. These aquatic Ludwigia species can impart severe ecological, economic, and human health impacts in aquatic ecosystems and threaten critical ecosystem functions. The authors expect these impacts to increase with greater global trade and projected climate change. This technical report presents an overview of the biology and ecology of these invasive plant species, along with select management case studies and research efforts. While the need for management approaches has become an important topic, little is known about the distribution of Ludwigia species and how they respond to varying environmental conditions in the U.S. Life history strategies and responses to environmental conditions vary among water primrose species. Therefore, species-specific management approaches may be required, and prevention and control strategies should be customized to the specific phase of the local invasion. This information is important for predicting further spread. Likewise, it is the foundation for risk assessments and effective management. This technical report proposes research priorities to improve understanding of the complexity of the biology and ecological invasion process of water primroses, and it provides resource managers with substantive recommendations for how to prevent and prioritize management of these aquatic weeds.
Water hyacinth (Eichhornia crassipes (Martius) Solms-Laubach) is a serious invasive weed in the Sacramento–San Joaquin River Delta of California. Three insects: Neochetina eichhorniae Warner and Neochetina bruchi Hustache (Coleoptera: Curculionidae) and Niphograpta (=Sameodes) albiguttalis (Warren) (Lepidoptera: Crambidae) were released during 1982–1987 at four locations for the biological control of water hyacinth. Observations in 1985 suggested that all three species had established. By 2002, water hyacinth populations in the Delta still required an aggressive chemical control campaign and the status of the biological control agents was in question. In late 2002, a field survey to determine the distribution and abundance of the released insects was performed. Water hyacinth plants were collected by boat in the main water channels and from land at smaller sloughs and examined for insects. In total, 27 sites with water hyacinth distributed across the Delta were examined of which 21 had weevils. Weevil abundance ranged from 0 to 10.9 weevils per plant, with an average of 0.93 (±0.47 SEM) adult weevils per plant. All weevils (n = 518) were identified as N. bruchi. No N. eichhorniae were recovered and no larvae or evidence of larval feeding by N. albiguttalis were observed. A total of 322 weevils were examined for microsporidia and none was found infected, indicating an infection rate of less than 1%. These results suggest that N. bruchi may be the only established biological control agent of water hyacinth in the Delta and that infection by microsporidia does not appear to be limiting its population abundance.
The invasive water hyacinth (Eichhornia crassipes) severely limits the ecosystem services provided by the Sacramento-San Joaquin River Delta in California, USA. As part of the biological control program in the Delta, two weevils, Neochetina bruchi and N. eichhorniae (Coleoptera: Curculionidae) and a moth, Niphograpta albiguttalis (Lepidoptera: Pyralidae), were released in the 1980s. An additional planthopper, Megamelus scutellaris (Hemiptera: Delphacidae) was released in 2011. We conducted monthly surveys for one year at 16 sites throughout 1,667 km² of the Delta to determine the resulting establishment, abundance and distribution of these introduced herbivores. Morphological identifications, and partial sequencing of the mitochondrial cytochrome oxidase subunit 1 gene determined that 96.6% of the examined weevils were N. bruchi. N. eichhorniae was only recovered from two sites in the southern Delta tributaries. Densities (larvae and adult weevils per destructively sampled plant) varied spatially and temporally. Peak mean densities (averaged across August-November) decreased with increasing distance from the original release sites. Peak mean densities ranged from 0.31 to 6.31 weevils per plant. Densities averaged across sites were the lowest in June 2015 (0.54 weevils), increasing in August to 5.35 weevils, and peaking in November at 6.22 weevils. The proportion of damaged leaf area from weevil feeding increased concomitantly with weevil densities. Although N. albiguttalis was not recovered, M. scutellaris remained established at its original release site but has not dispersed into the other surveyed regions. We propose hypotheses to explain patterns in species establishment and distribution, with potential mechanisms for improved future biological control.
Cyrtobagous salviniae is widely used in several countries including the United States, South Africa and Australia for biological control of Salvinia molesta Mitchell (Salviniales: Salviniaceae). Despite considerable success in tropical and subtropical regions, the effectiveness of C. salviniae on S. molesta is inconsistent in temperate regions, indicating the need for populations adapted to cooler climates. The objectives of this study were to determine the regions of South America that are climatically similar to S. molesta habitats in temperate Louisiana, conduct surveys for new provenances of C. salviniae in these regions, establish the phylogenetic relationships among C. salviniae provenances, and compare the cold tolerance of populations of C. salviniae from Louisiana, USA and the Lower Paraná-Uruguay Delta (LPUD), South America. Foreign exploration of the Lower Paraná-Uruguay Delta region resulted in the first record of C. salviniae in Uruguay, and revealed the most southern distribution of this species in Argentina and Uruguay. Phylogenetic identification of this provenance indicated that it is a different biotype to the one from Brazil which was first released in the United States in 2001. Analysis of the climate in south, central, and north Louisiana revealed colder and more frequent cold fronts in the north, likely contributing to winter population crashes. Regional differences in Louisiana thermal regimes indicate that there is a need for region specific management plans, such as the inclusion of cold tolerant C. salviniae populations in the management of S. molesta in temperate regions. The cold tolerance of C. salviniae populations from Louisiana and Lower Paraná-Uruguay Delta were compared by measuring survival at 0 °C, chill coma recovery times, and supercooling points (SCP). Survival at 0 °C was 1.5-times greater, mean chill coma recovery time was 1.8-times faster, and mean SCP was 1.2-times lower in the Lower Paraná-Uruguay Delta population compared to the Louisiana population. These findings show that the Lower Paraná-Uruguay Delta provenance should be considered for managing S. molesta in temperate regions. Besides host range tests on the Lower Paraná-Uruguay Delta biotype, cross breeding between the Louisiana and Lower Paraná-Uruguay Delta populations of C. salviniae should be investigated to determine the life histories of any possible hybrids.
An invasive grass, Arundo donax, occupies thousands of hectares of arid riparian habitat along the Rio Grande in Texas and Mexico, and has negative impacts on national security, water resources, and riparian ecosystems. The shoot-tip-galling wasp Tetramesa romana was released in 2009 between Brownsville and Del Rio, Texas, and has dispersed over 800 km along the river channel. Plots along the river were surveyed for shoot counts of arundo and all other plant species in 2016 at seven sites in regions in which prior studies had documented a 22% decline in arundo biomass (estimated from live shoot length) from 2007 to 2014. Estimated live biomass declined a further 32% between 2014 and 2016. Native plants accounted for 86% of the 44 species encountered in plots. Individual plots averaged five plant species, and arundo was most abundant in only 9 of 21 plots. Arundo live biomass and shoot density were negatively associated with plant diversity, indicating that live arundo interferes with germination and/or survival of other plant species. The proportion of dead shoots in plots, proportion of wasp-galled shoots, and density of exit holes made by emerging adult wasps per metre live main shoot length were positively associated with plant diversity in a combined model. Regressions indicated that the effects of wasp damage measures on diversity were mediated through their effects on main shoot mortality. By reducing live arundo biomass, the arundo wasp is fostering recovery of native plant communities at riparian sites along the Rio Grande.
Water hyacinth (Eichhornia crassipes (Martius) Solms-Laubach) is a non-native, invasive floating aquatic weed in the Sacramento San Joaquin Delta and associated river watersheds of northern California. Prior releases of biological control agents have not led to sustained control. The South American planthopper, Megamelus scutellaris Berg, 1883, permitted and released first in the southeastern U.S., was released at three sites in this region from 2011 to 2013, leading to establishment at one site in a pond off Willow Creek in Folsom in the American River watershed. Planthopper populations consisting of nymphs (two-Thirds or more of total counts) peaked in late summer each year between 2013 and 2015, reaching densities of six to nine planthoppers per plant by 2015. Megamelus scutellaris dispersed 50 m per year from the point of release between 2013 and 2015 and, based on degree-day estimation, were capable of producing four generations per year at the Folsom site. Proportion live leaves per plant declined by 27% in the Folsom pond between 2012 and 2015. In 2015, plants in the release pond had 40% less live above-water biomass than plants 200 meters away in a canal, into which planthoppers had dispersed in 2014-2015. This early impact of the planthopper could, however, be obscured by inter-Annual and within-site variability in plant growth. This study documents the first establishment of M. scutellaris on water hyacinth in the western U.S.
While biological invasions are increasing, in some cases exotic species exhibit an initial phase of population growth and spread, followed by a subsequent phase of natural decline. The light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), provides a unique opportunity to examine potential mechanisms for the natural suppression of an exotic insect species that has become established in coastal California. We recently discovered a microsporidian pathogen, Nosema fumiferanae postvittana, from E. postvittana in its novel range. In the laboratory, we examined the pathogenicity and latent period of this microsporidium, and in the field we determined its prevalence and intensity in five locations using quantitative real-time PCR (qPCR). In the laboratory, when comparing healthy larvae to larvae infected with up to 105 spores, we found a reduction in juvenile survivorship (from 100% to 26%), a prolongation of juvenile development time (of up to 9-10 days), a reduction in viable lifetime fecundity (from 788 to 1) and a reduction in the intrinsic rate of increase (from 0.18 to 0.008). The median lethal dose (LD50) was estimated to be 1.8 x 104 spores, and the mean latent period for infections with 103 spores was 12.67 days. Our field sampling revealed that E. postvittana populations have further declined from previously reported densities in San Francisco and Santa Cruz. We detected N. fumiferanae postvittana in all five locations with an overall prevalence of 5%, which did not appear to be influenced consistently by either host density or season. Mean microsporidian intensity in field-infected individuals was 226 spores. Although the laboratory results demonstrated the potential for host suppression, the field sampling indicated that the prevalence and intensity of microsporidian infection were too low to account for the continued decline in population densities of E. postvittana in coastal California.