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Coastal aquatic systems, including estuarine and marine nearshore environments, deserve our attention for three key reasons. First, healthy coastal systems provide homes and food for numerous plants and animals. Second, we use these systems extensively for commercial and recreational activities, and third, both our coastal and inland activities can pose threats to the health of coastal aquatic systems. One of the primary ways we threaten the health of coastal systems is through addition of nutrients. Nutrients occur naturally, and they support natural processes that make coastal systems unique. Unfortunately, our activities can increase nutrients to levels that cause undesirable changes. One of the most important ways we affect coastal waters is through activities on land that increase delivery of nutrients to the sea. This booklet provides information about the links between nutrients and the health of Florida’s coastal ecosystems. It uses definitions, conceptual models, and summaries of current knowledge to explain how coastal ecosystems function under natural conditions and how people are increasingly affecting coastal ecosystems. Sources of more information and suggestions for ways to help protect our precious coastal systems are included. Through a greater understanding of coastal systems, nutrients, and how people affect natural processes, each of us can make informed choices that reduce environmental damage.
Jennifer Hauxwell
Charles Jacoby
Thomas K. Frazer
John Stevely
We thank Bill Seaman for initiating this effort, and
Dorothy Zimmerman and Steve Kearl for guiding
it through the editorial process. Kenneth Louis
Clark waded through our suggestions and expertly
prepared the illustrations. Regina Cheong dealt with
the layout cheerfully, efficiently, and effectively.
The Authors
Jennifer Hauxwell
Department of Natural Resources Research Center
Wisconsin Department of Natural Resources
1350 Femrite Drive
Monona, Wisconsin 53716
Charles Jacoby
Department of Fisheries and Aquatic Sciences
University of Florida
7922 NW 71st Street
Gainesville, Florida 32653-3071
Thomas K. Frazer
Department of Fisheries and Aquatic Sciences
University of Florida
7922 NW 71st Street
Gainesville, Florida 32653-3071
John Stevely
Florida Sea Grant Extension Program
1303 17th Street West
Palmetto, Florida 34221-5998
This publication was supported by the National
Sea Grant College Program of the U.S. Department
of Commerce’s National Oceanic and Atmospheric
Administration (NOAA) under NOAA Grant No.
76 RG-0120, and by the UF/IFASCenter for
Natural Resources.The views expressed are those of
the authors and do not necessarily reflect the views
of these organizations.
Additional copies are available by contacting
Florida Sea Grant, University of Florida,
PO Box 110409, Gainesville, FL, 32611-0409,
(352) 392-5870.
Cover photo: Aerial of North Fork of the St. Lucie
River. Photo courtesy of South Florida Water
Management District.
All of us benefit from healthy coastal ecosystems. If we want our
recreational, commercial and other social benefits to continue, we have a
responsibility to protect the health of these systems. Balancing protection
and use of coastal systems creates some difficult decisions.
Florida Sea Grant recognizes the importance and complexity of such
decisions. As part of its efforts to enhance the practical use and conservation
of coastal and marine resources, it contributes to informed debate by
producing and distributing objective, valuable, and understandable
information. In order to produce such information, Florida Sea Grant
works with scientists from many collaborating organizations.
This brochure represents one of Florida Sea Grant’s efforts to generate
understanding and debate. In collaboration with researchers from the
University of Florida, we introduce an important and complex topic:
how nutrients function in coastal aquatic systems and how human activities
affect these natural cycles. Although a full explanation of nutrient
dynamics is beyond the scope of this brochure, it does help us understand
and manage these important phenomena.
Florida Sea Grant also recognizes that the issues related to use and
protection of Floridas coast extend beyond nutrients in coastal waters.
For that reason, it is producing other documents to accompany this one.
For example, Submarine Groundwater Discharge: An Unseen Yet Potentially
Important Coastal Phenomenon (SGEB-54), has been released. A citizen’s
guide to Florida’s estuaries is in preparation.
We ask that you read this material and pursue some of the additional
information. We also ask that you contact us with questions or comments.
Florida Sea Grant’s ability to achieve its objectives and our collective ability
to ensure sustainable use of coastal systems depend, in large part, on your
involvement and input.
Thank you for your time and interest.
Charles Jacoby
Extension Specialist, Estuarine Ecology
Leader, Florida Sea Grant Coastal Environmental and
Water Quality Design Team
Coastal aquatic systems, including estuarine and marine
nearshore environments, deserve our attention for
three key reasons. First, healthy coastal systems provide
homes and food for numerous plants and animals. Second,
we use these systems extensively for commercial and
recreational activities, and third, both our coastal and
inland activities can pose threats to the health of coastal
aquatic systems.
One of the primary ways we threaten the health of
coastal systems is through addition of nutrients. Nutrients
occur naturally, and they support natural processes that
make coastal systems unique. Unfortunately, our activities
can increase nutrients to levels that cause undesirable
changes. One of the most important ways we affect coastal
waters is through activities on land that increase delivery of
to the sea.
This booklet
provides information
about the links between nutrients and the health of
Florida’s coastal ecosystems. It uses definitions, conceptual
models, and summaries of current knowledge to explain
how coastal ecosystems function under natural conditions
and how people are increasingly affecting coastal ecosystems.
Sources of more information and suggestions for ways to help
protect our precious coastal systems are included. Through a
greater understanding of coastal systems, nutrients, and how
people affect natural processes, each of us can make
informed choices that reduce environmental damage.
Coastal systems include the water, bacteria, plants, and
animals found in bays, lagoons, estuaries, and
nearshore areas. All living things in coastal systems are
connected as illustrated by a food web (Figure 1). At the
base of coastal food webs, primary producers
(including seagrass, algae,
Figure 1: Simplified food web with connections among trophic levels. Primary producers (bacteria, phytoplankton, algae, and
seagrass) produce organic matter through photosynthesis. Primary consumers feed directly on bacteria and plants.
Secondary consumers eat primary consumers, and, in turn, are eaten by tertiary consumers. In truth, trophic connections
are often much more complex, with consumers feeding at several trophic levels. (Source: Florida Sea Grant)
and some species of bacteria) capture nutrients, sunlight,
and carbon dioxide (CO2) and use them to build new tissue
and release oxygen (O2) as a byproduct through a process
called photosynthesis. The production of new tissue is
referred to as primary production. Consumers, like us, feed
on primary producers or other consumers to gain energy
for survival, growth, and reproduction. All living things,
including primary producers, generate energy by consuming
organic material through a process called respiration. In
most cases, O2is used during the breakdown of organic
matter and release of stored energy.
In coastal systems, as in many other ecosystems,
primary production is often limited by the availability of
nutrients. Nutrients are chemical elements that influence
the productivity of all natural systems. Some elements that
are essential for the survival of primary producers and
consumers include nitrogen, phosphorus, potassium,
calcium, magnesium, sulfur, iron, manganese, copper,
zinc, molybdenum, sodium, cobalt, chlorine, bromine,
silicon, boron, and iodine. Primary producers extract these
essential nutrients directly from the environment. Like all
consumers, we get most of these elements from the food we
eat. In other words, we draw our nutrients from a food web.
Inputs of nutrients and other materials from the land
are a key feature distinguishing coastal systems from the
offshore oceanic environment. Although the exact offshore
boundary of a coastal system is difficult to pinpoint, we
can think of it as the place where inputs from the land no
longer have a significant influence. Inputs from the land
support rapid growth and reproduction of primary producers
and consumers making these areas among the most highly
productive in the world. Research shows that although
coastal waters represent only 10 percent of the total ocean
surface, they account for 20 percent of total primary
production and 50 percent of total fish production in the
oceans.3In large part, production of fish is high because of
the food and shelter provided by primary producers in these
relatively nutrient-rich waters (see shaded box, this page).
Coastal systems in Florida are affected both directly
and indirectly by many human activities. For example,
two key intertidal coastal habitats, mangroves and salt
marshes, have often been destroyed to make way for
housing and other developments. In Florida, dredging and
filling operations have destroyed over 23,000 acres out of
the state's 469,000 acres of mangroves.4,5 In addition,
Florida's coastal reefs have been damaged by direct contact
from boaters and divers, as well as through the indirect
effects arising from nutrient additions these systems.
Direct damage to coastal habitats is something to be
avoided, but in many areas, indirect effects from human
activities are more threatening.
In Florida and other places around the world, seagrass
meadows represent one of the most disturbed coastal habitats
and provide a good example of both direct and indirect
effects of human activity on habitat.
Under natural conditions, seagrasses often represent a
major submerged aquatic habitat (see shaded box,
page 3). Seagrass habitat has been lost from waters off
Florida, as well as from coastal waters around the world,
due to natural and human-induced disturbances.
Florida’s coastal environments represent one of the
state’s most distinctive and prominent features.
Maintaining the health of these environments is crucial.
Not only do they support fisheries, recreational activities,
and tourism, but they also provide the quality of life
that Floridians have come to enjoy. Did you know that
coastal environments:
border 35 of Florida’s 67 counties;
extend for 1,350 miles, which is longer than the
Atlantic coast from Georgia through Maine;
contain many habitats including coral reefs, sea
grasses, mangroves and wetlands;
shelter, during some part of their lives, grouper, sea
trout, redfish, oysters, clams, scallops, blue crabs,
lobsters, and other animals adding up to over
80 percent of the animals caught by recreational
and commercial fishers;
house 60 percent of Florida’s 16 million residents
within a band 10 miles wide;
draw 10 million tourists each year (twice the
number visiting inland parks)1;
support beach tourism, which generates
approximately $15 billion annually2; and
support marine fisheries worth $10 billion per year?
Natural disturbances that directly damage seagrasses
include hurricanes, earthquakes, ice scour, animals digging
through the substrate, animal grazing, and disease.
However, these pressures account for less than 20 percent
of the worldwide loss of seagrasses.6
Human activities also damage seagrass directly.
Dredging, construction of docks, mooring of boats,
harvesting of shellfish with rakes or trawls, and use of
motorboats in shallow waters all physically remove seagrass
meadows and created “scarred” areas. Seagrass meadows
with scars often suffer erosion and further loss of seagrass
during storms. In some meadows, scarring is a significant
problem, and overall approximately 6 percent of Florida's
seagrass meadows are scarred.7
In general, though, the links between many human
activities and effects on seagrasses involve multiple steps.
These effects are classed as indirect simply because
materials first need to be transported to the coast. One
example is herbicides carried to coastal waters, which can
then poison seagrasses. Another is sediment transported from
cleared land to coastal water, which can indirectly damage
seagrass by blocking out the light that it needs to grow.8
But it is the indirect effects of excess nutrient loading from
watersheds to coastal waters that are cited as the most
pervasive human impacts on coastal areas and on seagrass
habitat. 7,8,9-14 Around the world, increased nutrient
loading has accounted for 50 percent of the recorded
declines in seagrass, and this problem is growing.6
Algal blooms are one common result of excess nutrients
being delivered to coastal waters. Algae differ from
seagrasses and terrestrial plants in that they lack vascular
structures such as roots and stems. Algae may be microscopic,
such as single-celled phytoplankton, or easily seen by the
naked eye, macroscopic, and they may be free-living or
attached. Many species of algae have lower light requirements
than seagrasses, so algal growth is typically limited by the
availability of nutrients.
In Florida and many other places around the world,
undesirable increases in both small and large algae have
become more common due to increased nutrient loading
to coastal waters. Increased numbers of microalgae, or
phytoplankton, often give the water an opaque, greenish
appearance. Increased quantities of macroalgae may result
in piles of rotting “seaweed” on the bottom in coastal waters
or on beaches. Such undesirable increases in algae indicate
that the system is undergoing human-induced eutrophication,
or an unnaturally rapid buildup of organic matter.
Increased amounts of phytoplankton or macroalgae
may indirectly lead to the loss of seagrass (Figure 2).10, 12, 14
Increased nutrients in the water column have little direct
effect on seagrass growth because seagrass roots generally
absorb all the nutrients they need from within the sediment.
In contrast, fast-growing phytoplankton, algae that grow
on seagrass (epiphytes), and algae that grow on the bottom
Although they grow underwater, seagrasses are related
to flowering land plants, or angiosperms. Two key
characteristics that qualify seagrasses as angiosperms are:
1) a system of tubes, called a vascular system, which
transfers material within the plant, and 2) the production
of flowers as a way to reproduce sexually. Like many
land plants, seagrasses also reproduce vegetatively, with
new clones branching from an established plant.
As a result of both sexual and vegetative reproduction,
seagrasses often form extensive meadows. Along the
coasts of Florida, seagrass meadows cover more than
2.7 million acres.6These meadows serve several important
ecological roles, including:
1) fixing carbon dioxide (CO2) into new plant tissue
at twice the rate achieved by highly cultivated
crops, such as corn or rice. (Some seagrasses can
produce more than 800 grams of carbon per
square meter per year.);
2) providing food or shelter for thousands of marine
organisms (including invertebrates, fish, water
fowl, sea turtles, and manatees); and
3) preventing coastal erosion by binding sediments
with below-ground root and rhizome systems and
by reducing wave energy or the speed of currents
with above-ground leaf material.
In Florida, several seagrass species thrive in coastal
shallow waters, including:
1) Thalassia testudinum (turtle grass);
2) Halodule wrightii (shoal grass);
3) Syringodium filiforme (manatee grass);
4) Halophila engelmannii (star grass);
5) Halophila johnsonii (Johnson’s seagrass);
6) Halophila decipiens (paddle grass); and
7) Ruppia maritima (widgeon grass).
affect not only the habitat and food available to animals
but also the “biogeochemistry” of coastal waters (see box
on this page, for an example). Under normal, low nutrient
conditions, O2concentrations rise during the day as primary
producers photosynthesize and produce O2and fall during
the night as all organisms respire or use O2to generate
energy. Generally, O2concentrations remain high enough
for animals to survive. However, excessively low O2
conditions arise when respiratory use of O2by aquatic
communities (primary producers and consumers) exceeds
the sum of O2release during photosynthesis by primary
producers and passive diffusion of O2from air to water.
Under such circumstances, certain invertebrates and fishes
may suffocate.
In particular, increased amounts of algae caused by
increased nutrient loading can cause O2concentrations to
(benthic macroalgae) are often nutrient limited, and they
respond to higher nutrient loads by growing faster.12, 14,
15–18 Increased amounts of these producers remove a large
percentage of the light that would otherwise have been
available for seagrass photosynthesis, and the seagrasses are
“starved” of the light they need to survive. Lack of suitable
light, in fact, is probably the major cause of damage to
seagrasses. This “shading” effect can also damage habitats
other than seagrass. For example, overgrowth of algae
leading to reduced light can eventually kill corals.
The indirect effects of increased nutrients do not stop
at the loss of seagrass habitat because that loss often means
that an important habitat for some animals is degraded. As
seagrasses are lost, animal numbers may decline because
they rely on seagrass for protection from predators or
because they rely on plants and animals in seagrass meadows
for food. Examples of animals affected in this manner
include certain species of invertebrates (such as amphipods,
isopods, shrimps, and snails) that are important in the diet
of several species of fishes. Furthermore, juvenile scallops
often settle directly upon the leaves of seagrasses, and the
loss of seagrass may contribute to diminished numbers of
scallops along Florida’s Gulf coast. For a few species,
increased primary productivity may actually represent an
increase in food. For example, growth rates of clams like
the quahog, Mercenaria mercenaria, may increase in systems
with higher nutrient loads because extra nutrients stimulate
production of their food – phytoplankton and particles
that are filtered from the water column.19
The shift from seagrass to a system with more algae
and more organic matter (a more eutrophic system) can
High concentrations of nutrients, particularly nitrogen,
carried by the Mississippi River to the Gulf of Mexico
have stimulated substantial increases in algal production.
Much of the excess algae settles to the seafloor where it is
degraded by bacteria. The degradation process consumes
O2, and greater inputs of algae cause more degradation and
more loss of O2. Extensive loss of oxygen, a biogeochemical
change, creates a “dead zone” in the northern Gulf of
Mexico.20–22 In essence, the “dead zone” contains no liv-
ing plants or animals. During the past 20 summers, the
dead zone has increased in size to cover an area the size
of New Jersey.
Figure 2: Drawings of a) a healthy seagrass bed and b) an unhealthy seagrass bed shaded and overgrown by phytoplankton and algae as a
consequence of increased nutrients. (Source: Florida Sea Grant)
most abundant chemical element in living tissue, behind
oxygen, carbon, and hydrogen. Phosphorus is also a key
component in DNA, and it is found in adenosine
triphosphate (ATP), a molecule that is important in energy
transfer and storage in living cells.
Because N and P largely control primary productivity,
we are primarily concerned about their addition to our
coastal waters. Phosphorus is often the “limiting”
nutrient in freshwater environments, meaning the
addition of P stimulates primary productivity. Nitrogen
is more frequently limiting in marine environments.
However, many exceptions to this pattern can be found
along the coast of Florida and elsewhere around the
world. Some of Florida’s coast has sediments that are
rich in calcium carbonate (CaCO3). Phosphate (PO4),
the form of P typically used by primary producers,
binds to these sediments and becomes less available. So,
in Florida’s coastal waters, either N or P may limit
growth. We focus here on N as an example of how
humans affect the natural cycles of key nutrients.
fluctuate greatly. Although algae contribute O2when they
photosynthesize, their respiration and the respiration of
bacteria that degrade dead algae can use more O2than is
produced. On sunny days, photosynthetic production is
usually greater than respiratory demand and there may be
no problem. However, a series of cloudy days can lead to
less photosynthesis and less O2production, so that respira-
tory demand can exceed O2availability. In these situations,
low oxygen concentrations (hypoxia) or loss of all oxygen
(anoxia) causes the death of invertebrates and fishes.
Of all the essential nutrients, nitrogen (N) and
phosphorus (P) are the two nutrients that most often
limit the growth of primary producers. Nitrogen is a key
component in 1) chlorophyll, the green pigment in primary
producers that absorbs sunlight during photosynthesis,
2) amino acids, the building blocks of proteins, and
3) genetic material, including deoxyribonucleic acid (DNA)
and ribonucleic acid (RNA). Nitrogen ranks as the fourth
Figure 3: Simplified diagram of the nitrogen cycle showing some key sources of nitrogen (household septic systems; fertilizer from lawns,
gardens and agriculture; waste from livestock; point source discharges from wastewater treatment plants and industry; exhaust from
cars; and emissions from industry), some key mechanisms that transport nitrogen (rain, runoff, leaching, groundwater, and rivers),
and some key processes that transform nitrogen (nitrogen fixation, denitrification and photosynthesis). (Source: Florida Sea Grant)
Many essential nutrients are made available to primary
producers through natural weathering of the earths
crust. Unlike other nutrients, N is most abundant as di-
nitrogen gas (N2). In fact, this gas comprises 78 percent of
the air we breathe. Although it is abundant, N still limits
the growth of primary producers, because most plants and
algae cannot directly use N2. Primary producers cannot
break the very strong and stable chemical bonds between
the atoms in nitrogen gas.
So, how do primary producers get N (Figure 3)?
Lightning can produce the immense, focused energy needed
to break the bonds in N2, and it produces nitrite (NO2),
nitrate (NO3), and ammonia (NH3). In addition, living
organisms called nitrogen-fixers can convert N2to
ammonium (NH4), the form of N most readily taken up by
primary producers. In terrestrial environments, free-living
microbes and symbiotic bacteria in peas, alfalfa, soybeans
and other legumes perform this function, and in marine
environments, cyanobacteria (commonly called blue-greens)
fix N. Plants can also take up NO3, but this process requires
energy (the use of ATP), and NO3must be converted to
NH4before the N can be used in protein synthesis. Other
natural sources of N are the production of animal waste,
natural burning of organic matter or fossil fuels, and the
decomposition of organic material, such as the decay of
dead algae.
There are three primary sources of human-derived N:
1) wastewater, 2) fertilizer, and 3) atmospheric pollution
(Figure 3). Nitrogen from these sources is delivered to
coastal regions in three ways: 1) point source inputs of
water, such as rivers or wastewater outlets; 2) nonpoint
source inputs of water, such as direct surface runoff or
groundwater that has percolated through the soil, into an
aquifer and out to coastal waters as springs or diffuse
plumes; and 3) direct atmospheric deposition (Figure 3).23
Nitrogen has always been delivered from the land to the
sea, and this transfer helps make coastal waters productive.
However, increases in N that can be used by primary
producers (bioavailable N) arising from increases in human
activities pose a threat to healthy coastal systems (see
shaded box, on this page). For example, an estimated
37 percent of the world’s population currently lives within
62 miles of the coastline, and we can expect 75 percent
of the U.S. population to live within 47 miles of the
coastline by the year 2010.24, 25 Increased coastal populations
with their residential, commercial, industrial, and agricultural
activities have increased the delivery of anthropogenic, or
human-derived, N and P to estuaries via point and nonpoint
sources of wastewater and fertilizer. Air pollution from
industry and cars also contributes N to coastal systems via
direct deposition into the sea or via deposition to land and
subsequent transport by groundwater or surface runoff.26
The pressure on coastal waters in states such as Florida
is increased by our country’s agricultural practices. A
majority of crop production in the United States occurs in
the inland, midwestern states, with food being transported
to people in coastal states. This translocation means that
coastal populations import N in the form of food from
inland regions and export N in the form of waste to coastal
Over the past century, human activities have more than
doubled the rate at which atmospheric N2or organically-
bound N is converted to biologically available forms
(Table 1). The transfer of atmospheric N has increased due
to 1) increased cultivation of nitrogen-fixing crops such as
peas, alfalfa, and soybeans, and 2) production of artificial
fertilizers. The technology for the industrial production of
artificial fertilizers was developed in Germany during World
War I, after Fritz Haber synthesized the base for them,
NH3, by combining N2and hydrogen gas (H2) at high
temperature and pressure. We have increased the transfer of
organically-bound N by 1) burning fossil fuels, such as coal
and oil, and 2) burning or clearing land.
Table 1.Global nitrogen fixation in terrestrial environments.
Data are summarized from Vitousek et al. (1997).23
Input New nitrogen
(million metric tons per year)
bacterial fixation 90–140
lightning <10
Total 100–150
bacterial fixation 90–140
lightning <10
cultivation of N-fixing crops 40
fertilizer production 80
burning fossil fuels >20
burning or clearing existing land 70
Total 310–360
'Preserve wetland buffers or green space and submerged
aquatic vegetation associated with coastlines, rivers,
and streams. Important “buffer” habitats include salt
marshes and mangroves. Plants in these systems
remove N and P as they grow. In addition, wetland
sediments are good sites for “denitrification”, a
process in which bacteria convert nitrate (NO3, a
biologically available form of nitrogen) into nitrogen
gas (N2) that enters the atmosphere and becomes
less available to coastal primary producers. Efforts
should also be made to preserve submerged vegetation,
such as seagrasses, because this vegetation also
removes nutrients. For example, people should
avoid direct removal of submerged vegetation by
dredging or raking, and they can minimize indirect
loss due to shading by building docks at a proper
height, orienting them north–south rather than
east–west, and spacing their boards.27
'Limit the use of fertilizers on residential and commercial
lawns and landscaping. When fertilizing, it is
important to minimize quantities and avoid
fertilizing before heavy rains. Storm water runoff
can transport fertilizer intended for yards directly to
coastal waters. A University of Florida extension
program, Florida Yards and Neighborhoods, provides
information to homeowners on environmentally
friendly landscaping that reduces storm water runoff,
decreases nonpoint source pollution, conserves water,
enhances wildlife habitat, and creates attractive
landscapes (see the web site listed below for further
'Manage storm water runoff. Diverting runoff to
retention ponds or other temporary storage areas
minimizes direct input of nutrients to coastal waters.
Minimizing the extent of impervious surfaces while
maximizing the cover of natural vegetation also
reduces the amount of storm water runoff delivered
to coastal waters.
'Use better septic systems. Repairing old or leaking
septic systems or replacing them with more efficient
systems will help keep nutrients from reaching
groundwater or runoff. For example, denitrifying
systems rely on the natural microbial process of
“denitrification” to convert nitrate (NO3), which
can enter groundwater and move to coastal waters,
into nitrogen gas (N2) that is instead released to the
systems. Overall, coastal systems receive a disproportionately
high load of nitrogen generated by the activities of people.
Scientists and environmental managers are attempting to
predict the level of N loading that a system can
accommodate. The concept of assimilative capacity or total
maximum daily load (TMDL) refers to this threshold.
Unfortunately, the nutrient load that causes undesirable
changes in coastal ecosystems, the threshold loading, is not
a simple thing to predict. Uncertainty arises from many
causes, including the complex and interactive cycles of N and
other nutrients, geological variation among sites, seasonal
variations in rainfall and sunlight, and the influence of
isolated events. For example, a system may exceed its
assimilative capacity when unusually large rainfalls deliver
high pulses of nutrients. Once the system has undergone a
change, it may not readily revert back to its former state.
Scientists and managers are also protecting our coastal
systems by monitoring their status and adjusting
management efforts accordingly. Monitoring often involves
measurements of nutrient concentrations in the water
column, but these concentrations change rapidly and vary
from place to place. Such variation makes it difficult to
accurately identify long-term increases. As a “safety net”,
some monitoring includes bioindicators and bioassays that
focus on changes in natural vegetation, changes in growth
rates, or changes in the type of N found in plants. The
hope is that these “indicators” will integrate short-term
pulses to provide an improved view of long-term trends.
First and foremost, we can limit nutrient inputs to
coastal waters. Examples of actions that might decrease
nutrient loading to coastal waters include:
'Limit urban development, especially along shorelines.
Increases in development result in increased nutrient
loads via many pathways. For example, more
development and more people often mean that
coastal waters will receive more nutrients from septic
systems, wastewater treatment plants, or fertilized
lawns and landscaping. Development increases the
amount of “impervious surfaces” (such as roads,
driveways, or roofs) that generate nutrient-rich,
storm water runoff, and it also diminishes forests
and natural “green spaces” that slow runoff and
remove nutrients.
1) Florida Coastal Management Program. 2000. Florida
assessment of coastal trends. Florida Department of
Community Affairs, Tallahassee, FL. pp. 1–148.
2) Florida Coastal Management Program. 1996. Florida State
of the Coast Report, Preparing for a Sustainable Future.
Florida Department of Community Affairs. Tallahassee,
Florida. 28 pp.
3) Ryther, J.H. 1969. Photosynthesis and fish production in
the sea. Science 166: 72–76.
4) Humphreys, J., S. Franz, B. Seaman, and J. Potter. 1993.
Florida’s estuaries: a citizen’s guide to coastal living and
conservation. Florida Sea Grant Publication SGEB–23.
Florida Sea Grant, Gainesville, Florida. 25 pp.
5) Department of Environmental Protection, Florida Marine
Research Institute web site:
6) Short, F.T. and S. Wyllie-Echeverria. 1996. Natural and
human-induced disturbance of seagrasses. Environmental
Conservation 23: 17–27.
7) Sargent, F.J., T.J. Leary, D.W. Crewz, and C.R. Kruer. 1995.
Scarring of Florida’s seagrasses: assessment and management
options. FRMI Technical Report RT-1. Florida Marine
Research Institute, St. Petersburg, Florida. 37 pp. plus
8) Kemp, W.M., R.R. Twilley, J.C. Stevenson, W.R. Boynton,
and J.C. Means. 1983. The decline of submerged vascular
plants in upper Chesapeake Bay: Summary of results
concerning possible causes. Marine Technology Society
Journal 17: 78–89.
9) Group of Experts on the Scientific Aspects of Marine
Pollution (GESAMP). 1990. The state of the marine
environment. Blackwell Scientific Publications, Oxford.
146 pp.
10) Short, F.T., D.M. Burdick, J.S. Wolf, and G.E. Jones. 1993.
Eelgrass in estuarine research reserves along the East Coast,
U.S.A., Part I: Declines from pollution and disease and Part
II: Management of eelgrass meadows. National Oceanic
and Atmospheric Administration, Coastal Ocean Program
Publication, Rockville, Maryland. pp. 1–83, M1–M24.
'Improve sewage treatment plants. Different levels of
wastewater treatment result in different levels of
nutrient removal. Tertiary treatment results in the
most complete removal of nutrients. Improved
wastewater treatment can dramatically affect coastal
ecosystems. For example, two decades ago, seagrass
coverage in Tampa Bay was less than 30 percent of
historical levels. The loss was largely attributed to
nutrient loading. In the 1980s, improving treatment
of inputs from domestic and industrial point sources
created a 50 percent reduction in N loading. Water
quality in Tampa Bay has steadily improved, and, as
a result, coverage of seagrass has expanded by about
12 percent.28
'Support efforts to improve our knowledge. We can all
support research and monitoring that will supply
information that is critical to future improvements.
In short, we can strive to know more so that we can
make informed decisions.
Understanding the response of coastal primary produc-
ers to nutrient loading, the interactions among these pro-
ducers, and the indirect effects of changes in these pro-
ducers on other elements of coastal systems is crucial for
preventing loss and damage in coastal environments.
Preventing loss and damage is the most effective form of
management, because restoration of coastal habitats will be
far more costly and difficult if not impossible.29–31
At this time, it is difficult to accurately predict the
amount of nutrients that can be safely added to coastal
waters. Small-scale experiments have shown the existence of
links between nutrient supply, algal production, and loss of
seagrass habitat.16, 17, 32–36 However, numerical relationships
that specify the exact nutrient load resulting in undesirable
responses at the estuary scale are only available for a few
systems around the world. We need to conduct large-scale
experiments and undertake monitoring to validate
extrapolations and predictions made from small-scale
experiments and to accurately estimate the rate at which
nutrients are being added to Florida’s coastal waters.
Monitoring not only helps us understand the interactions
between human activities and coastal systems at larger scales,
it also gives us early warnings of problems.
Scientists, managers, and community members need to
work together to develop research initiatives and monitoring
programs that detect and predict small, relevant changes
caused by increased nutrient loads. Such experimental and
observational research would allow us to further understand
the relationship between nutrient loading and changes to
coastal habitats. This understanding can be used to formulate
effective, but not unnecessarily restrictive, policies for
managing human activities.
11) National Research Council. 1994. Priorities for Coastal
Ecosystem Science. National Academy Press. Washington,
D.C. 118 pp.
12) Valiela, I., J. McClelland, J. Hauxwell, P.J. Behr, D. Hersh,
and K. Foreman. 1997. Macroalgal blooms in shallow
estuaries: Controls and ecophysiological and ecosystem
consequences. Limnology and Oceanography 42: 1105–1118.
13) United States Geological Survey. 1999. The quality of our
nation’s waters – nutrients and pesticides. United States
Geological Survey Circular 1225. pp. 1–82.
14) Hauxwell, J., J. Cebrián, C. Furlong, and I. Valiela. 2001.
Macroalgal canopies contribute to eelgrass (Zostera marina)
decline in temperate estuarine ecosystems. Ecology 82:
15) Duarte, C. 1995. Submerged aquatic vegetation in relation
to different nutrient regimes. Ophelia 41: 87–112.
16) Short, F.T., D.M. Burdick, and J.E. Kaldy III. 1995.
Mesocosm experiments quantify the effects of
eutrophication on eelgrass, Zostera marina. Limnology and
Oceanography 40: 740–749.
17) Taylor, D., S. Nixon, S. Granger, and B. Buckley. 1995.
Nutrient limitation and the eutrophication of coastal
lagoons. Marine Ecology Progress Series 127: 235–344.
18) Hauxwell, J., J. McClelland, P.J. Behr, and I. Valiela. 1998.
Relative importance of grazing and nutrient controls of
macroalgal biomass in three temperate shallow estuaries.
Estuaries 21: 347–360.
19) Weiss, E. 2001. The effect of N loading on the growth rates
of quahogs and softshell clams through changes in food
supply. Boston University, M.A. Thesis. 52 pp.
20) Rabalais, N.N., W.J. Wiseman, Jr., R.E. Turner, D. Justic,
B.K. Sen Gupta, and O. Dortch. 1996. Nutrient changes
in the Mississippi River and system responses on the
adjacent continental shelf. Estuaries 19: 386–407.
21) Malakoff, D. 1998. Death by suffocation in the Gulf of
Mexico. Science 281: 190–192.
22) Ferber, D. 2001. Keeping the stygian waters at bay. Science
291: 968–973.
23) Vitousek, P.M., J. Aber, R.W. Howarth, G.E. Likens, P.A.
Matson, D.W. Schindler, W.H. Schlesinger, and G.D.
Tilman. 1997. Human alteration of the global nitrogen
cycle: causes and consequences. Ecological Applications
7: 737–750.
24) Williams, S.J., D. Dodd, and K.K. Gohm. 1991. Coasts in
crisis. United States Geological Survey Circulation 1075.
32 pp.
25) Cohen, J.E., C. Small, A. Mellinger, J. Gallup, and
J.D. Sachs. 1997. Estimates of coastal populations.
Science 278: 1211.
26) Corbett, R.D., W.C. Burnett, and J.P. Chanton. 2001.
Submarine groundwater discharge: an unseen yet
potentially important coastal phenomenon. Florida Sea
Grant, Gainesville, Florida. 6 pp.
27) Burdick, D.M. and F.T. Short. 1998. Dock design with the
environment in mind: minimizing dock impacts to eelgrass
habitats (CD-ROM). New Hampshire Sea Grant, program
number: UNHMP-V-SG-98-18. National Sea Grant
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28) Johansson, J.O.R. and H.S. Greening. 2000. Seagrass
restoration in Tampa Bay: a resource-based approach to
estuarine management. pp. 279–293. In Bortone, S.A.
(ed.), Seagrasses: Monitoring, Ecology, Physiology, and
Management. CRC Press, New York.
29) Harrison, P.G. 1990. Variations in success of eelgrass
transplants over a five-year period. Environmental
Conservation 17: 157–163.
30) Davis, R.C. and F.T. Short. 1997. Restoring eelgrass,
Zostera marina L., habitat using a new transplanting
technique: the horizontal rhizome method. Aquatic Botany
59: 1–15.
31) Davis, R.C., F.T. Short, and D.M. Burdick. 1998.
Quantifying the effects of green crab damage to eelgrass
transplants. Restoration Ecology 6: 297–302.
32) Twilley, R.R., W.M. Kemp, K.W. Staver, J.C. Stevenson,
and W.R. Boynton. 1985. Nutrient enrichment of estuarine
communities. 1. Algal growth and effects on production of
plants and associated communities. Marine Ecology
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33) Burkholder, J.M., H.B. Glasgow, Jr., J.E. Cooke. 1994.
Comparative effects of water-column nitrate enrichment on
eelgrass Zostera marina, shoalgrass Halodule wrightii, and
widgeongrass Ruppia maritima. Marine Ecology Progress
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34) Neckles, H.A., R.L. Wetzel, and R.J. Orth. 1993. Relative
effects of nutrient enrichment and grazing on
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State of Florida
National Sea Grant
Florida Sea Grant
Florida Yards and Neighborhoods
Apalachicola (Apalachicola, FL) NOAA
National Estuarine Research Reserve
Rookery Bay (Naples, FL) NOAA National
Estuarine Research Reserve
Florida EPA National Estuary Program
Indian River Lagoon
Tampa Bay
Sarasota Bay
Charlotte Harbor
Florida Water Management Districts
St. Johns River Water Management District
Southwest Florida Water Management District
South Florida Water Management District
Northwest Florida Water Management District
Suwannee River Water Management District
Florida Center for Environmental Studies
Florida Oceanographic Society
University of Hawaii (seagrass pictures)
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Pollution (GESAMP). 1990. The state of the marine
environment. Blackwell Scientific Publications, Oxford.
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waters - nutrients and pesticides. U.S. Geological Survey
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P.A. Matson, D.W. Schindler, W.H. Schlesinger, and
G.D. Tilman. 1997. Human alteration of the global nitrogen
cycle: causes and consequences. Issues in Ecology 1: 1–15.
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America, 2010 Massachusetts Avenue NW, Suite 400,
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on-line at:
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Ecosystem Science. National Academy Press. Washington,
D.C. ISBN: 0-309-05096-0. On line at:
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Science Serving
Florida’s Coast
Florida Sea Grant College Program
University of Florida
PO Box 110409
Gainesville, FL 32611-0409
(352) 392-5870
October 2001
Reviewed October 2008
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Full-text available
Seagrass restoration and management following substantial water quality improvements
Full-text available
An ecosystem-level experiment was conducted to identify the nutrient most Limiting to productivity and biomass in the marine lagoons of the northeast United States. Mesocosms containing a complex of species characteristic of shallow coastal marine environments were enriched with P alone, N alone, or combined N plus P, at loadings typical of highly enriched natural lagoons. The mesocosms showed significant responses to enrichment with N alone but not P alone, indicating limitation by N. Enrichment with N alone caused increased water column concentrations of chlorophyll a and particulate nitrogen (PN), increased water column daytime net production (NP), and increased rates of growth of juvenile winter flounder. It also caused eelgrass beds and mats of drift macroalgae to decline, apparently in response to phytoplankton shading. Comparison of the N-alone and combined N+P treatments indicated that when enriched with N alone, the Limitation of the systems shifted to P limitation of total system metabolism and of phytoplankton production and standing crop, and to light limitation of eelgrass and macroalgal growth. In the combined N+P mesocosms, water column concentrations of chlorophyll a, PN, and particulate P, rates of total system and water column NP and night-time respiration, and growth rates of juvenile winter flounder and killifish were all increased relative to the N-alone mesocosms. Declines of eelgrass and macroalgae were also more severe.
Full-text available
In an experimental mesocosm system during late summer-fall, we examined shoot production by eelgrass Zostera marina without nitrate additions (generally with ambient water-column concentrations < 2 muM NO3--N) versus production by eelgrass that previously had been exposed to low nitrate enrichment (pulsed additions of 5 muM NO3--N d-1 to the water for 12 wk during an unusually cool spring season). During late summer-fall, the previously enriched plants were subjected to higher nitrate loading (10 muM NO3--N d-1 for 14 wk), while control plants were maintained without nitrate additions as in spring. We also compared shoot production in fall by recent field transplants of Z. marina, Halodule wrightii, and Ruppia maritima with and without additions of 10 muM water-column NO3--N d-1. Low water exchange (10 % d-1) was used to simulate conditions in sheltered embayments or lagoons, and light reduction from high tide was simulated by covering the mesocosms with neutral-density screens that reduced incident light by 30 % for 3 h d-1 on a rotating schedule. Shoot production by both enriched and unenriched Z marina was comparable during the spring low-level NO3--N exposure. However, eelgrass enriched with nitrate in both spring and fall attained significantly lower shoot production than control plants without enrichment. This decrease, unrelated to light reduction from algal growth, suggests a direct adverse effect of long-term water-column nitrate exposure on Z marina. The more recent transplants of eelgrass without prior enrichment history also showed a trend for decreased lateral growth under moderately elevated nitrate. In contrast, H. wrightii was slightly stimulated and R. maritima was highly stimulated by water-column nitrate relative to growth of controls. By the end of the fall experiment, Z marina (+/- NO3--N), H. wrightii (+/- NO3--N), and unenriched R. maritima had increased shoot densities by less-than-or-equal-to 50%, whereas nitrate-enriched R. maritima increased shoot production by > 300 %. The data indicate that H. wrightii or R. maritima could be established successfully by transplanting efforts as a management strategy in nitrate-enriched waters where eelgrass meadows have disappeared. Unlike Z marina, these species apparently have developed physiological mechanisms to more effectively control nitrate uptake and metabolism.
Full-text available
Loss of eelgrass (Zostera marina) habitat from temperate estuaries worldwide often coincides with increased macroalgal accumulations resulting from increased delivery of anthropogenic nitrogen. We conducted macroalgal enclosure/exclosure experiments during summer 1998 within eelgrass populations in two estuaries of Waquoit Bay, Massachusetts, USA, to evaluate how increased macroalgal biomass affects density, recruitment, growth rate, and production of eelgrass. One estuary featured a low nitrogen loading rate and sustained a relatively pristine eelgrass population with a 2 cm high macroalgal canopy. The other estuary had a sixfold higher nitrogen loading rate and a declining eelgrass population with a 9 cm high macroalgal canopy. Experimental units were 1 × 1 m plots of eelgrass fenced within 50 cm high plastic mesh that excluded or included macroalgae at canopy heights ranging from 0 to 25 cm. In both estuaries, rates of eelgrass loss increased, largely a result of decreased recruitment, and growth rates decreased (due to decreased rates of leaf appearance) with increasing macroalgal canopy height. Aboveground summer production in both estuaries decreased exponentially as macroalgal canopy heights increased. We conclude that macroalgal cover is a proximate cause for loss of eelgrass in the higher N estuary since, upon removal of macroalgae, we observed an increase in shoot density, a 55% increase in summer growth, and a 500% increase in summer aboveground net production. Based on summer growth data and density of shoots in our experimental plots the following spring, we suggest that the negative impacts of macroalgal canopies persist, but also that eelgrass recovery upon removal of macroalgae may be possible. To identify the mechanisms by which macroalgae potentially inhibit eelgrass production, we measured changes in nutrient and oxygen concentrations resulting from macroalgal canopies and estimated the relative importance of summer standing stocks of phytoplankton, epiphytes, and macroalgae to potential shading of eelgrass in both estuaries. We document both (1) unfavorable biogeochemical conditions (lowered redox conditions and potentially toxic concentrations of NH4+) imposed by the presence of macroalgal canopies and (2) potential light limitation of eelgrass by standing stocks of producers in the higher N estuary, with estimates of light reduction via macroalgae numerically more important than light sequestration by phytoplankton and epiphytes for newly recruiting shoots. Increased macroalgal biomass associated with increased nitrogen loading to estuaries can lead to eelgrass disappearance, and we identify an approximate 9-12-cm critical macroalgal canopy height at which eelgrass declines.
This paper provides a summary of research conducted to investigate possible causes of the decline in abundance of submerged aquatic vegetation beginning in the late 1960s. Three factors are emphasized: runoff of agricultural herbicides; erosional inputs of fine-grain sediments; nutrient enrichment and associated algal growth. The results are synthesized into an ecosystem simulation model which demonstrated relative potential contributions, where nutrients greater than sediments greater than herbicides. Other factors and mechanisms are also discussed along with resource managements options.
Macroalgal blooms arc produced by nutrient enrichment of estuaries in which the sea floor lies within the photic zone. We review fcaturcs of macroalgal blooms pointed out in recent literature and summarize work done in the Waquoit Bay Land Margin Ecosystems Research project which suggests that nutrient loads, water residcncc times, presence of fringing salt marshes, and grazing affect macroalgal blooms. Increases in nitrogen supply raise macroalgal N uptake rates, N contents of tissues, photosynthesis-irradiance curves and P,,,.,, and accelerate growth of fronds. The resulting increase in macroalgal biomass is the macroalgal bloom, which can displace other estuarine producers, Fringing marshes and brief water residence impair the intensity of macroalgal blooms. Grazing pressure may control blooms of palatable macroalgac, but only at lower N loading rates. Macroalgal blooms end when growth of the phytoplankton attenuates irradiation reaching the bottom. In cstuarics with brief water rcsidencc times, phytoplankton may not have enough time to grow and shade macrophytcs. High phytoplankton division rates achieved at high nutrient concentrations may compensate for the brief time to divide before cells arc transported out of the estuary. Increased N loads and associated macroalgal blooms pervasively and fundamentally alter estuarinc ecosystems. Macroalgae intercept nutrients regenerated from sediments and thus uncoupIe biogeochemical sedimentary cycles from those in the water column. Macroalgae take up so much N that water quality seen:? high even where N loads are high. Macroalgal C moves more readily through microbial and consumer food webs than C derived from seagrasscs that were replaced by macroalgae. Macroalgae dominate 0, profiles of the water columns of shallow estuaries and thus alter the biogeochemistry of the sediments. Marc frequent hypoxia and habitat changes associated with macroalgal blooms also changes the abundance of bcnthic fauna in affected estuaries. Approaches to rcmediation of the many pervasive cffccts of macroalgal blooms riced to include interception of nutrients at their watcrshcd sources and perhaps removal by harvest of macroalgae or by increased flushing. Al- though we have much knowledge of macroalgal dynamics, all such management initiatives will require additional information.