Review of the potential for low impact seagrass restoration in Aotearoa New Zealand.

Abstract

The purpose of this report was to examine the potential for low impact forms of seagrass restoration in Aotearoa New Zealand, with a focus on the Whakatū/Nelson region. Seagrass restoration methods and decision-making frameworks used internationally were reviewed. We also provide information to support some of these decisions including the environmental requirements, natural recovery timeframes and genetic diversity of Zostera muelleri. We also highlight the role of local iwi in restoration efforts.

Full text

REPORT NO. 3697
REVIEW OF THE POTENTIAL FOR LOW IMPACT
SEAGRASS RESTORATION IN AOTEAROA NEW
ZEALAND

CAWTHRON INSTITUTE | REPORT NO. 3697 SEPTEMBER 2021
REVIEW OF THE POTENTIAL FOR LOW IMPACT
SEAGRASS RESTORATION IN AOTEAROA NEW
ZEALAND
DANA CLARK*, ANNA BERTHELSEN*
*Joint first authors
Prepared for Nelson City Council
Envirolink advice grant 2146 NLCC119
CAWTHRON INSTITUTE
98 Halifax Street East, Nelson 7010 | Private Bag 2, Nelson 7042 | New Zealand
Ph. +64 3 548 2319 | Fax. +64 3 546 9464
www.cawthron.org.nz
REVIEWED BY:
Paul Gillespie
APPROVED FOR RELEASE BY:
Grant Hopkins
ISSUE DATE: 17 September 2021
RECOMMENDED CITATION: Clark D, Berthelsen B 2021. Review of the potential for low impact seagrass restoration in
Aotearoa New Zealand. Prepared for Nelson City Council. Cawthron Report No. 3697. 53 p. plus appendices.
© COPYRIGHT: This publication may be reproduced in whole or in part without further permission of the Cawthron Institute
or the Copyright Holder, which is the party that commissioned the report, provided that the author and the Copyright Holder
are properly acknowledged.

CAWTHRON INSTITUTE | REPORT NO. 3697 SEPTEMBER 2021
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EXECUTIVE SUMMARY
Seagrass meadows (Zostera muelleri) are one of Aotearoa New Zealand’s most
important coastal ecosystems. They provide crucial habitat for fish and other animals,
mitigate climate change through carbon sequestration and improve coastal water
quality by taking up nutrients and contaminants, and trapping sediments.
Unfortunately, Aotearoa New Zealand, like many places worldwide, has experienced
extensive seagrass decline, primarily due to physical disturbance and deteriorating
water quality. Concern about the loss of seagrass meadows has prompted global
efforts to facilitate natural recovery of, and actively restore, seagrass. In Aotearoa
New Zealand, active seagrass restoration has been trialled by transplanting seagrass
from wild donor meadows. However, removing plants from donor sites causes
damage to wild seagrass meadows. Lower impact methods of seagrass restoration
are available but have yet to be used in Aotearoa New Zealand.
The purpose of this report was to examine the potential for low impact forms of
seagrass restoration in Aotearoa New Zealand, with a focus on the Whakatū/Nelson
region. Seagrass restoration methods used internationally were reviewed. These
included the facilitation of natural recovery through the reduction or removal of
stressors (passive restoration) and active restoration using seagrass seeds or plants.
Seagrass plants can be obtained by harvesting them from wild seagrass meadows,
collecting naturally uprooted seagrass fragments, germinating seeds or propagating
plants in a nursery. Plants are then transplanted as intact sods of seagrass or bare-
rooted plants, with various methods available to fix them to the seafloor and/or shelter
them from environmental conditions and animals.
We then evaluated the potential for low impact forms of seagrass restoration in
Aotearoa New Zealand. Seed-based methods have not been trialled in Aotearoa New
Zealand because it was previously thought that flowering (required for seed
production) in Z. muelleri occurred infrequently. However, recent research indicates
flowering might be more common than realised, although seagrass seeds have been
rarely encountered. Restoration using naturally uprooted seagrass fragments,
germinated seeds or seagrass plants propagated in nurseries is likely to have little to
no impact on wild seagrass populations. However, these approaches often involve
growing seagrass in nurseries, which is expected to cost more than harvesting wild
plants from donor meadows.
To improve restoration success and to conduct restoration in an ethical manner, a
suite of decisions needs to be made prior to commencing restoration activities. We
review available decision-making frameworks and provide information to support
some of these decisions. This information includes the environmental requirements,
natural recovery timeframes and genetic diversity of Z. muelleri. We also highlight the
role of local iwi in restoration efforts.

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Within the Whakatū/Nelson region, a high-level assessment indicated that Te Tai o
Aorere/Nelson Haven, Kokorua Inlet and Wakapuaka/Delaware Inlet show the most
potential for seagrass restoration. The next steps to assess feasibility of low impact
seagrass restoration include further trial of seed-based (e.g., identifying a source of
seeds, germinating seeds and cultivating seedlings in nurseries) and plant-based
(e.g., cultivating and propagating seagrass plants in nurseries) restoration methods.
Steps to initiate decision making for seagrass restoration in the Whakatū/Nelson
region include developing a decision-making framework and partnerships with local
iwi, and identification of suitable restoration sites and methods.

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TABLE OF CONTENTS
1. INTRODUCTION ........................................................................................................... 1
2. SEAGRASS RESTORATION METHODS ...................................................................... 5
2.1. Facilitating natural recovery (passive restoration) .......................................................................................... 5
2.2. Active restoration using seagrass seeds ........................................................................................................ 6
2.2.1. Collecting, processing and storing seeds from the wild ............................................................................. 7
2.2.2. Spreading and planting seeds ................................................................................................................... 8
2.3. Active restoration using seagrass plants ........................................................................................................ 9
2.3.1. Obtaining seagrass plants for transplantation ........................................................................................... 9
2.3.2. Nursery systems for storage, cultivation and propagation of seagrass ................................................... 13
2.3.3. Transplanting seagrass plants ................................................................................................................ 15
3. SEAGRASS RESTORATION IN AOTEAROA ...............................................................18
3.1. Previous active restoration efforts ................................................................................................................ 18
3.2. Information relevant to seed-based restoration potential .............................................................................. 20
3.3. Potential for low impact forms of seagrass restoration ................................................................................. 21
3.3.1. Passive restoration .................................................................................................................................. 22
3.3.2. Seed-based restoration ........................................................................................................................... 22
3.3.3. Obtaining plants for transplanting ............................................................................................................ 22
3.3.4. Transplanting .......................................................................................................................................... 23
4. DECISION MAKING FOR SEAGRASS RESTORATION ..............................................27
4.1. Decision making frameworks and considerations ......................................................................................... 27
4.2. Information to support seagrass restoration decisions in Aotearoa New Zealand ........................................ 28
4.2.1. Key environmental requirements ............................................................................................................. 28
4.2.2. Timeframes for natural recovery.............................................................................................................. 32
4.2.3. Seagrass genetics ................................................................................................................................... 33
4.2.4. Iwi considerations .................................................................................................................................... 34
4.3. Seagrass restoration potential for Whakatū/Nelson estuaries ...................................................................... 34
4.3.1. Waimeha/Waimea Inlet ........................................................................................................................... 35
4.3.2. Te Tai o Aorere/Nelson Haven ................................................................................................................ 35
4.3.3. Wakapuaka/Delaware Inlet ..................................................................................................................... 36
4.3.4. Kokorua Inlet ........................................................................................................................................... 36
5. NEXT STEPS ................................................................................................................37
5.1. Trial of low impact seagrass restoration methods ......................................................................................... 37
5.2. Steps to initiate decision making for seagrass restoration ............................................................................ 37
6. ACKNOWLEDGEMENTS .............................................................................................39
7. REFERENCES .............................................................................................................39
8. APPENDICES ...............................................................................................................54
LIST OF FIGURES
Figure 1. A subtidal seagrass meadow at Great Mercury Island (left) and an intertidal seagrass
meadow at Shakespeare Bay (right), Aotearoa New Zealand (Photos: Dana Clark and
Anna Berthelsen, Cawthron Institute). ................................................................................ 1

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Figure 2. Bubble shell hiding within a seagrass meadow at Slipper Island, Aotearoa New
Zealand (Photo: Dana Clark, Cawthron Institute). .............................................................. 2
Figure 3. Photographs showing scouring of subtidal seagrass surrounding swing moorings at
Slipper Island (left), and vehicle tracks on intertidal seagrass in Wakapuaka/ Delaware
Inlet (right), Aotearoa New Zealand (Photos: Dana Clark and Anna Berthelsen,
Cawthron Institute). ............................................................................................................. 3
Figure 4. Flowering stages of Zostera marina: 1) styles erect from the spadix, 2) styles bend
back after pollination, 3) pollen is released from the anthers, 4) seed maturation, 5)
seeds are released. Image taken from Infantes and Moksnes (2018). .............................. 7
Figure 5. Fragment of Zostera muelleri (Photo: The Marine Life Database 2020). ......................... 11
Figure 6. The morphology of Zostera marina seedlings. A, seed; B, after 1 d; C, after 2 d; D,
after 1 week (arrows show cotyledon and leaf); E, after 2 weeks; F, after 3 weeks; G,
after 4 weeks; and H, after 5 weeks. (Photos: Pan et al. 2011). ...................................... 12
Figure 7. Growing seagrass (Zostera muelleri) in a simple nursery system in Aotearoa New
Zealand (Photos: Cade 2016). .......................................................................................... 14
Figure 8. Intact sods being transported (left) and in-situ at the restoration site (right) in the Avon-
Heathcote seagrass transplant experiment (Photos: Gibson & Marsden 2016). ............. 19
Figure 9. Seagrass flowering in Zostera muelleri showing protruding stigma to catch pollen for
fertilisation (Photo: Tracey Burton, from de Kock et al. 2016). ......................................... 20
LIST OF TABLES
Table 1. Summary of the pros and cons of different seagrass restoration methods for
application in Aotearoa New Zealand. .............................................................................. 25
Table 2. Optimal environmental conditions for Zostera muelleri plants based on studies in
Aotearoa New Zealand (modified from Schwarz et al. 2005; Zabarte-Maeztu 2021).
Seagrass also needs to be free of physical impacts that could cause damage. .............. 31
LIST OF APPENDICES
Appendix 1. A code of conduct for seagrass (Posidonia oceanica) restoration. .................................. 54
Appendix 2. A decision-making strategy for transplanting Posidonia oceanica and other
seagrasses. ....................................................................................................................... 55

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1. INTRODUCTION
Seagrasses are flowering marine plants that form extensive meadows in both
intertidal and shallow subtidal coastal environments. These meadows commonly
occur in sheltered, soft-sediment areas, away from strong currents and wave action
(e.g., estuaries; Hemminga & Duarte 2000; Spalding et al. 2003). Aotearoa New
Zealand has only one species of seagrass, Zostera muelleri
1
(Figure 1). It colonises
intertidal areas and grows subtidally where water clarity permits (Turner & Schwarz
2006).
Figure 1. A subtidal seagrass meadow at Great Mercury Island (left) and an intertidal seagrass
meadow at Shakespeare Bay (right), Aotearoa New Zealand (Photos: Dana Clark and
Anna Berthelsen, Cawthron Institute).
Seagrass meadows are one of the world’s most valuable coastal ecosystems
(Costanza et al. 1997), offering an array of ecosystem services that benefit society
and the environment (Nordlund et al. 2016). A single meadow creates a variety of
habitats (e.g., on and between the plants, within the sediment and overlying water
column) amid an often homogenous soft-sediment seafloor (Spalding et al. 2003). As
such, they are hotspots of biodiversity, providing food, shelter and refuge for a range
of organisms (e.g., macroinvertebrates, epiphytes, fish, birds; Hemminga & Duarte
2000; Figure 2). As one of the most productive ecosystems on Earth (Duarte &
Chiscano 1999), seagrass meadows help to regulate the climate by transforming
carbon dioxide into biomass as they grow (Duarte et al. 2010; Lavery et al. 2013).
Additional carbon storage capacity arises from the ability of seagrass meadows to trap
1
Previously known as Z. capricorni or Z. novaezelandiae (Jacobs et al. 2006) and commonly referred to as
eelgrass, karepō, nana, rehia and rimurehia.

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carbon-rich sediments from adjacent habitats. By stabilising sediments and taking up
nutrients and contaminants (e.g., metals; Birch et al. 2018) from the water as they
grow, seagrasses also act as natural filters, thereby improving or maintaining coastal
water quality (Short & Short 1984).
Figure 2. Bubble shell hiding within a seagrass meadow at Slipper Island, Aotearoa New Zealand
(Photo: Dana Clark, Cawthron Institute).
Seagrass meadows are often used as indicators of ecological health, due to their
sensitivity to a range of stressors (e.g., Martínez-Crego et al. 2008). The sheltered
coastal areas that seagrasses occupy are often close to areas heavily used by
humans, making this species particularly exposed to anthropogenic threats. As a
result, seagrass meadows are disappearing at a rapid rate worldwide, with at least
29% of global seagrass area lost already (Waycott et al. 2009). Aotearoa New
Zealand is no exception; significant declines in seagrass extent occurred during the
1920s and 1970s (Inglis 2003) and continue today in many places (e.g., Park 2016).
Consequently, the conservation status of Z. muelleri in Aotearoa New Zealand is ‘At
Risk – Declining’ (de Lange et al. 2018).
Seagrass loss is often the result of poor water quality driven by human activities (e.g.,
coastal development and poor land management; Cullen-Unsworth & Unsworth
2013). Increased sediment and nutrient loads can reduce water clarity and stimulate
phytoplankton, macroalgae and epiphytes that compete with seagrass for light (Short
& Wyllie-Echeverria 1996). The release of toxic compounds in coastal waters (e.g.,

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metals, herbicides, pesticides) can also inhibit seagrass growth and photosynthesis
(Bester 2000; Macinnis-Ng & Ralph 2002; McMahon et al. 2005; Ambo-Rappe et al.
2011). Activities such as dredging, coastal development, anchoring, scouring from
moorings, land reclamation and vehicle impact can cause physical damage to
seagrass meadows (e.g., Walker et al. 1989; Erftemeijer & Robin Lewis 2006;
Ceccherelli et al. 2007; Šunde et al. 2017; Figure 3). Other threats to seagrass include
severe storms, overgrazing and/or competition from natural or introduced species,
and fungal wasting disease (Matheson et al. 2009). Climate change could also place
further pressure on seagrass meadows through exposure to more frequent/intense
storm events, poorer water quality, extreme temperatures, and a reduction in suitable
habitat due to sea level rise and coastal squeeze (Unsworth et al. 2019).
Figure 3. Photographs showing scouring of subtidal seagrass surrounding swing moorings
at Slipper Island (left), and vehicle tracks on intertidal seagrass in Wakapuaka/
Delaware Inlet (right), Aotearoa New Zealand (Photos: Dana Clark and Anna
Berthelsen, Cawthron Institute).
Concern about the loss of seagrass meadows has prompted efforts globally to
facilitate natural recovery and actively restore seagrass (Orth & McGlathery 2012;
Paulo et al. 2019; Tan et al. 2020; Boudouresque et al. 2021). In Aotearoa New
Zealand, improvements in environmental conditions have led to natural recovery of
seagrass meadows in some places (e.g., Whangarei Harbour; Matheson et al. 2017;
Morrison 2021). Active restoration efforts have also been trialled by transplanting
seagrass from wild donor meadows. However, removing plants from donor sites
places pressure on the wild seagrass meadows that remain. Increasingly,

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communities and environmental managers are becoming less comfortable with the
idea of removing plants from these meadows. Restoration using seeds, or
transplantation of naturally uprooted seagrass fragments, germinated seeds or
seagrass propagated in a nursery could allow for large scale efforts without damaging
wild seagrass meadows.
Nelson City Council (NCC) is exploring options for seagrass restoration in their region.
Decline in seagrass meadow extent has been recorded (or inferred) in the four main
estuaries in Whakatū/Nelson (Stevens & Robertson 2015; Stevens & Forrest 2019a,
2019b; Stevens et al. 2020). To avoid placing further pressure on remaining seagrass
meadows, NCC is interested in the potential to use low impact methods (i.e., those
that cause no or minimal damage to wild seagrass) to actively restore seagrass
meadows in the region. Thus, the purpose of this report is to examine the potential to
apply these low impact seagrass restoration methods in Aotearoa New Zealand, with
a focus on the Whakatū/Nelson region:
• We first summarise seagrass restoration methods used internationally and in
Aotearoa New Zealand.
• We describe in further detail previous active restoration efforts and information on
flowering and seed production for Z. muelleri in Aotearoa New Zealand and
evaluate the potential for low impact seagrass restoration.
• We review decision making frameworks for restoration and provide information to
support some of these decisions in Aotearoa New Zealand. This information
includes the environmental requirements, natural recovery timeframes and genetic
diversity of Z. muelleri, and iwi considerations relevant to seagrass restoration.
• We then evaluate the potential to apply low impact seagrass restoration methods
in Whakatū/Nelson estuaries, followed by an outline of the next steps to achieve
this goal.

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2. SEAGRASS RESTORATION METHODS
Ecosystem restoration as defined here can take two forms: passive restoration where
the reduction or removal of an anthropogenic stressor(s) enables the system to
naturally recover, or active restoration where humans directly facilitate recovery
following the reduction or removal of an anthropogenic stressor(s) (Elliott et al. 2007).
In the case of active restoration, reseeding or transplanting is used to establish
seagrass at a restoration site. Because the recovery of impacted seagrass via natural
processes can take longer than a decade, active restoration methods could be an
essential component of a restoration strategy (Balestri et al. 2011). Below, we
summarise passive and active seagrass restoration methods based on information in
the literature, and comment on method success, resources required and likely impact
to wild seagrass populations and the wider environment. Restoration methods used
should cater for the seagrass species (and ideally population) that is being targeted.
This is because species of seagrass can vary widely in their biological characteristics
(e.g., reproductive traits; Orth et al. 2000; Kendrick et al. 2012), especially at the
genus or higher level. To ensure that the information summarised is most relevant, we
focus on species of the genus Zostera as this is the genus of seagrass present in
Aotearoa New Zealand. However, examples of restoration efforts relating to other
seagrass taxa are provided where they offer additional useful information. Information
specific to Aotearoa New Zealand is discussed in Section 3.
2.1. Facilitating natural recovery (passive restoration)
Reducing or removing stressors impacting seagrass populations can passively
facilitate natural recovery of seagrass (Paling et al. 2009). The benefits of this can
extend beyond seagrass health to the wider ecosystem and human wellbeing (e.g.,
through improved water quality). Once environmental conditions are suitable for
seagrass (refer Section 4.2.1 for further details), natural recovery can occur through
(1) the expansion of the meadow due to rhizome growth, (2) the influx of naturally
uprooted vegetative fragments from nearby meadows and (3) the germination of
seeds produced by plants within the meadow or from nearby meadows
(Boudouresque et al. 2021). As well as being a method of restoration in its own right,
passive restoration is also a prerequisite for active restoration as the cause of
seagrass decline needs to be established and reduced (or removed altogether) for
active restoration to be successful (van Katwijk et al. 2009; refer to Section 4).
Actions taken to alleviate or remove physical stressors include the installation of
environmentally friendly vessel moorings that reduce damage to seagrass (Ferretto et
al. 2021), boating restrictions to reduce seagrass scarring from propellors (Dawes et
al. 1997), and the recent banning of anchoring by large ships on seagrass meadows
in the French Riviera (Boudouresque et al. 2021, https://medposidonianetwork.com/).
Improvements in water quality, at both localised (e.g., around a wastewater pipe) and

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larger (i.e., estuary- or bay-wide) scales can also facilitate seagrass recovery. For
example, reduced catchment nutrient loads, sewage treatment and disuse of sewage
outfalls has led to the natural recovery of seagrass meadows (including Zostera
species) in Europe, the Mediterranean, South Africa and the USA (Bryars &
Neverauskas 2004; Leschen et al. 2010; Riemann et al. 2016; Boudouresque et al.
2021).
Passive restoration efforts worldwide have had varying levels of success (Tan et al.
2020). Poor success has been associated with factors such as failure to ensure the
original stressor was alleviated, abiotic and biotic interactions (Moksnes et al. 2008;
Valdemarsen et al. 2010) and propagule supply limitations (Orth et al. 1994; Kendrick
et al. 2012). Timeframes for natural recovery of Zostera can be relatively slow,
especially if large spatial scales are involved (refer Section 4.2.2). The financial cost
and effort of passive restoration can vary substantially. Alleviating (or removing) local-
scale impacts to seagrass meadows is likely to cost less than implementing larger-
scale changes. For example, prohibiting vehicle access to an estuary is likely to be
less expensive than improving water quality through catchment-scale land
management changes, although in some cases both may need to happen for natural
recovery to occur. It can also be more difficult to get support (e.g., from a societal or
political perspective) for large-scale changes. In some cases, passive seagrass
restoration is might not be possible, for example, if the original loss was due to land
reclamation for development.
Promoting positive biological interactions could help to improve seagrass restoration
success (Tan et al. 2020), both for natural recovery via passive restoration as well as
active restoration. For seagrass, biological interactions can occur between microbial
communities, plants, shellfish and coral reefs (Tan et al. 2020) as well as herbivores
(Tol et al. 2021). For example, in a highly exposed habitat, seagrass planted within a
mussel bed had better survival than transplants located 60 m away from the mussel
bed (Bos & van Katwijk 2007). Negative biological interactions can also occur. For
example, Zostera meadows can be replaced by mangroves (Cann et al. 2009) and
cordgrass (Cottet et al. 2007; Roberts & Pullin 2008), although environmental
conditions likely play a role in facilitating this in some cases.
2.2. Active restoration using seagrass seeds
Sexual reproduction via flowering and seed production represents an important
mechanism for maintaining dispersal, connectivity, and genetic diversity in seagrass
populations (Kendrick et al. 2012). Seagrass seeds collected from wild seagrass
populations and spread or planted directly onto a restoration site is a low impact
restoration technique (Boudouresque et al. 2021). Furthermore, large scale seed-
based methods could provide an economically feasible source of plant material
(Busch et al. 2010; Statton et al. 2013), although seed handling and storage can

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require considerable effort (Pickerell et al. 2006).The various steps in seed-based
restoration, which include seed collection, processing and storage, followed by
spreading or planting, are described in more detail below.
2.2.1.
Collecting, processing and storing seeds from the wild
Seed-based restoration first requires the collection of seeds from donor meadows.
Seagrass seeds attached to floating shoots can also disperse (Stafford-Bell et al.
2016), therefore, beach-cast wrack may be a potential source of seeds for restoration.
Seagrass flowering and seed production has various stages (e.g., Figure 4), following
seasonal patterns controlled by light, temperature and nutrients (Dos Santos &
Matheson 2017). An in-depth understanding of sexual reproduction for the target
seagrass population is important given the variation that can occur between species
and populations (Kendrick et al. 2012; Dos Santos & Matheson 2017). For Z. marina,
the collection of flowering shoots with maturing seeds for restoration has traditionally
been carried out by hand (e.g., via snorkelling or SCUBA; Busch et al. 2010; Orth &
McGlathery 2012). Mechanical harvesting of Z. marina seeds with a barge can be
used to improve collection efficiency and still has a low impact on the donor meadow
(Marion & Orth 2010). However, collection rates depend on seagrass productivity and
it is currently unknown if mechanical harvesting is worthwhile for Australian and New
Zealand species (Tan et al. 2020). Once collected, flowering shoots can be
suspended above tanks until the seeds are shed, which is followed by processing to
separate the seeds from leaf and stem matter (Marion & Orth 2010; Tanner & Parham
2010).
Figure 4. Flowering stages of Zostera marina: 1) styles erect from the spadix
2
, 2) styles bend back
after pollination, 3) pollen is released from the anthers
3
, 4) seed maturation, 5) seeds are
released. Image taken from Infantes and Moksnes (2018).
2
A spike of minute flowers closely arranged around a fleshy axis and typically enclosed in a modified leaf called a
spathe.
3
Part of the stamen (pollen producing part of the flower) that produces the pollen.

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The seeds of most seagrass species (including Zostera) have a dormant stage (Orth
et al. 2000), where they spend time in the sediment before germinating. For example,
along the mid-Atlantic coast of North America, Z. marina seeds are shed during spring
and summer but generally do not germinate until the following autumn (Tanner &
Parham 2010). For species with seed dormancy, collected seeds can be processed
and then stored for a period of time so that spreading or planting aligns with the
season in which seed germination naturally occurs. For Z. marina restoration in North
America, seeds were processed by separating them from the senescing shoots and
then stored in flow-through or recirculating seawater tanks until spreading (Moore et
al. 1993; Marion & Orth 2010; Tanner & Parham 2010; Orth & McGlathery 2012).
Ideal seed storage time has been studied for various Zostera species; for example, it
was recommended that storage of Z. marina shoots should not exceed 40 days to
allow optimal release of viable seeds (Infantes & Moksnes 2018). The environmental
conditions (e.g., salinity and temperature) under which Zostera seeds are stored can
influence the length of time that they stay viable during storage (Tanner & Parham
2010; Yue et al. 2019). Viable seeds can also be sorted prior to spreading to improve
germination success. Viability of Zostera seeds has been determined by assessing
their firmness and fall velocity (Marion & Orth 2010; Orth & McGlathery 2012), by
using a vertical flume (Infantes & Moksnes 2018) or by using tetrazolium
4
(Conacher
et al. 1994). Seed germination (and seedling recruitment) can also be enhanced in
situ (refer Section 2.3.1).
2.2.2.
Spreading and planting seeds
Once seagrass seeds are collected, processed and stored, they can then be spread
or planted at the restoration site where they can germinate and develop into plants.
Seeds have been traditionally spread by hand, which has proven to be successful
even at large scales. For example, 10 million Z. marina seeds were broadcast by
hand from a moving boat into four coastal lagoons over two decades in a successful
restoration project in Virginia, USA (Orth et al. 2020). Not all seed-based restoration
efforts are successful though. A Swedish trial, where reproductive shoots were stored
within mesh bags, recorded very low Z. marina seedling establishment, although high
seedling growth rate occurred in those that did survive (Eriander et al. 2016). A study
on dispersal found that seed-based restoration of Z. muelleri was more likely to
succeed in areas of existing seagrass rather than in disturbed areas where seagrass
is no longer present (Macreadie et al. 2014).
Various other techniques have been trialled for planting Z. marina seeds, with differing
levels of success. These include using planting seeds into peat pots, broadcasting
seeds and then covering the area with burlap, placing seeds in burlap bags and then
burying the bags in the field, deploying seed-bearing shoots in buoys, and using a
mechanical planter (Harwell & Orth 1999; Orth et al. 2006; Pickerell et al. 2006; Orth
et al. 2009; Marion & Orth 2010; Orth & McGlathery 2012). Dispenser injection
4
Tetrazolium is a chemical commonly used to check seed viability.

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seeding, where seeds and sediment are mixed and injected into the substrate (Tan et
al. 2020) could be a useful method to test in Aotearoa New Zealand. This approach
was less labour intensive than other seeding techniques and could be more
successful in intertidal areas because the seeds are less likely to get washed away.
Seed-based restoration can provide an opportunity for community groups to work
together, for example, in Western Australia, fishers, powerboat owners and scientists
have together sown over 200,000 viable Posidonia australis seeds (Sinclair et al.
2021).
2.3. Active restoration using seagrass plants
Vegetative development via rhizomes is the main mechanism for seagrass expansion
(Marbà & Duarte 1998). Thus, active seagrass restoration often involves planting
seagrass at restoration sites with the goal of establishment and expansion via clonal
reproduction. Seagrass plants for active restoration can be obtained by 1) harvesting
wild plants from donor sites, 2) collecting naturally uprooted seagrass fragments, 3)
germinating seeds, or 4) propagating plants in a land-based nursery (i.e., land-based
laboratory aquaria/tanks or aquaculture systems). These plants are then transplanted
to a restoration site (sometimes following storage or cultivation in a nursery) using a
variety of methods.
2.3.1.
Obtaining seagrass plants for transplantation
Harvesting wild plants from donor sites
Seagrass plants can be obtained from donor sites in the wild, by digging up intact
clods
5
of seagrass (including shoots, rhizomes, leaf blades and roots) or by collecting
‘bare-rooted’ plants
6
(rhizome sections with leaf-bearing shoots and rhizomes/root
nodes; Ganassin & Gibbs 2008). Taking seagrass from wild meadows is the most
common method of obtaining seagrass plants for restoration (Ferretto et al. 2021) and
has been used globally in restoration efforts for a range of seagrass species (Fonseca
et al. 1998; Paling et al. 2001; Tan et al. 2020; Boudouresque et al. 2021). Clods are
generally preferred over bare-rooted plants because the root and rhizome systems
remain intact and the original sediment accompanies the plants to the new location
(Fonseca et al. 1998). Furthermore, clods provide greater anchorage than bare-rooted
plants (Paling et al. 2001).
Obtaining seagrass plants from wild meadows has the potential to impact donor
populations through physical damage that causes fragmentation of the meadow and
inhibition of natural re-growth (Bull et al. 2004). Physical disturbance could also
5
Similar terms to ‘clods’ include plugs, cores, sods, matte, blocks and turf. Terminology may depend on the size
and shape of the sediment clod.
6
Similar terms to ‘bare-rooted plants’ include sprigs, shoots, cuttings and rhizomes. Terminology may depend on
the specific part of the plant being used.

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increase seagrass susceptibility to diseases (e.g., fungal wasting disease
Labyrinthula; Turner & Schwarz 2006). Due to the potential for negative impacts, the
conservation status of some seagrass species restricts this method of collection in
certain countries (e.g., Australia, Europe; Coles & Fortes 2001; Boudouresque et al.
2012). Harvesting wild plants from donor meadows is generally more expensive than
collecting seeds because it involves more effort (Fishman et al. 2004; Tanner &
Parham 2010). Furthermore, it restricts, rather than increases, genetic diversity in
restored populations (Williams 2001).
Collecting naturally uprooted seagrass fragments
Bare-rooted seagrass plants that have detached naturally (commonly referred to as
‘seagrass fragments’) due to natural (e.g., storms, strong winds) or anthropogenic
(e.g., anchoring, dredging) processes can be collected and used as material for
seagrass restoration, thereby avoiding the need to damage donor meadows. They are
often found washed ashore on beaches adjacent to seagrass meadows and are
inexpensive to collect. Collection success can be increased by identifying favourable
collection conditions based on factors such as winds and tides (e.g., Australia,
Europe; Ferretto et al. 2021). Balestri et al. (2011) calculated that up to 150 fragments
could be collected by one person in an hour following an intense storm. This approach
is conducive to citizen-scientist engagement (e.g., Ferretto et al. 2021), which can
increase the number of fragments collected, reduce restoration costs, raise
awareness and create a sense of community ownership in restoration programmes.
The length of time that seagrass fragments remain viable after detachment depends
on the seagrass species and the environmental conditions (e.g., temperature, UV
damage). In an Australian study, 50% of Z. muelleri fragments remained buoyant after
5 weeks of floating and most of these were viable (Stafford-Bell et al. 2015). Z. marina
fragments have been shown to remain viable for 12 weeks, although their
reestablishment capacity is significantly reduced after six weeks (Ewanchuk &
Williams 1996). To be capable of producing new shoots and roots, seagrass
fragments should ideally have at least one shoot attached to a rhizome, with larger
fragments expected to grow faster because they have more meristems
7
(Figure 5;
Balestri et al. 2011). An Australian study found that 3-11% of beach-cast Z. muelleri
fragments were capable of regrowth (Stafford-Bell et al. 2015). Fragments must be
collected before they dry out and kept in suitable conditions until planting (refer
Section 2.3.2). Although storage of fragments can increase handling costs and reduce
the survival of fragments, this process naturally selects the most viable fragments for
transplantation and means fragments do not need to be re-planted on the same day
as collection, allowing for transplantation in larger batches during a favourable season
(Balestri et al. 2011; Ferretto et al. 2021).
7
A region of plant tissue, found chiefly at the growing tips of roots and shoots and in the cambium, consisting of
actively dividing cells forming new tissue.

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Figure 5. Fragment of Zostera muelleri (Photo: The Marine Life Database 2020).
Following collection, fragments can then be propagated in a laboratory or nursery or
directly planted at a restoration site. As the survival of stored fragments often
decreases over time, fragments planted within a few months of collection may have
the greatest success (Balestri et al. 2011). Direct transplantation of fragments has
been successful for Z. muelleri. For example, Zhou et al. (2014) reported > 95%
survival after three months and no significant differences in shoot density, height or
aboveground biomass compared to a wild meadow after three years. However, in a
different study 100% of transplanted Z. nigricaulis fragments died within 100 days,
possibly because of the timing of the transplantation (Thomson et al. 2015).
Germinating seeds
An alternative to directly spreading or planting seagrass seeds is to germinate seeds
under cultured conditions and then on-grow the seedlings (Figure 6) prior to
transplantation. Benefits of this approach include reducing physical damage to donor
meadows (compared with harvesting wild plants), increasing plant survival rates, and
facilitating genetic diversity in the restored seagrass population (Tan et al. 2020).
Barriers to using this method include limited seed availability for some species (Orth
et al. 2000) and the resources required to maintain a nursey system (refer Section
2.3.1) and harvest the plants in preparation for transplantation (Tanner & Parham
2010). Germinating Z. muelleri seeds under cultured conditions has also received little
attention overseas, although in Australia the practicality of germinating seeds of this
species has been recently assessed (Tan et al. 2020). However, large-scale seedling
culture has been successfully demonstrated for Z. marina, where 1500 shoots were
grown from seed and transplanted to a restoration site in Chesapeake Bay, USA
(Tanner & Parham 2010). It took between 70 to 100 days to germinate the seeds and

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grow the plants to a size large enough to transplant (Tanner & Parham 2010), with
cultured plants performing better at the restoration site than plants taken from wild
donor meadows (Tanner et al. 2010).
On smaller scales, seed germination and seedling growth and survival in cultured
conditions has been demonstrated in experimental trials for several Zostera species
(Abe et al. 2009; Tanner & Parham 2010; Kishima et al. 2011; Pan et al. 2011; Wang
et al. 2016; Xu et al. 2016; Cumming et al. 2017). Germination was found to be
influenced by several environmental variables including salinity, temperature,
dissolved oxygen, sediment type, and seed burial depth (Moore et al. 1993; Orth et al.
2000; Abe et al. 2010; Kishima et al. 2011; Wang et al. 2016; Cumming et al. 2017).
Survival and growth of cultured seedlings was also influenced by temperature and
salinity, as well as light and sediment nutrient enrichment (Abe et al. 2010; Xu et al.
2016).
Figure 6. The morphology of Zostera marina seedlings. A, seed; B, after 1 d; C, after 2 d; D, after 1
week (arrows show cotyledon
8
and leaf); E, after 2 weeks; F, after 3 weeks; G, after 4
weeks; and H, after 5 weeks. (Photos: Pan et al. 2011).
8
Embryonic leaf in seed-bearing plants (the first leaves to appear from a germinating seed).

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Actions can also be taken to enhance natural seagrass germination and seedling
survival in the wild or to collect germinating seeds and transplant them once they
develop into plants. Methods trialled include laying a sheet, bag or fibre membrane on
the seafloor to help prevent loss of seeds and seedlings (Seddon et al. 2005; Tanner
2015; Sousa et al. 2017).
Propagating seagrass plants
Seagrass shoots can be propagated vegetatively from adult plants that have either
been transplanted from wild donor sites or grown in a nursery from seeds or naturally
uprooted vegetative fragments. Conceivably, propagation of seagrass plants in land-
based nurseries (refer Section 2.3.2) could provide a continuous supply of seagrass
plants for low-impact restoration, as occurs for terrestrial plants. This could also create
opportunities to selectively breed cultivars with faster growth rates, greater tolerance
to environmental stressors (e.g., turbidity, nutrient loading, climate change impacts) or
disease resilience. For example, Tanner and Parham (2010) observed a 19-fold
increase in the number of shoots from Z. marina plants grown in the nursery over six
months. By harvesting shoots and replanting these into the nursery at lower densities,
they were able to maintain seagrass in a nursery system for four years. Similarly,
dividing P. oceanica fragments grown in a nursery for three years doubled the number
of transplants for restoration (Balestri et al. 2011). Emerging propagation techniques
(e.g., micropropagation and callus induction) that are commonly used for terrestrial
crops are also beginning to be developed for seagrasses (Hamill & Sumby 2002;
Ailstock & Shafer 2006; Tigani 2007; O'Brien 2019). These approaches offer the
potential to rapidly propagate disease-free plants from limited donor material and
require less space than traditional methods of propagation (O'Brien 2019). Although
propagation (via traditional or emerging methods) has the potential to provide a
continuous supply of seagrass plants for restoration, this clonal method of
reproduction is likely to result in low genetic diversity (and thus resilience) in the
restored population. Genetic diversity can be maintained by starting clones from
seeds, or plants/fragments collected from several seagrass populations or populations
with high genetic diversity (Tanner & Parham 2010; refer Section 4.1.3).
2.3.2.
Nursery systems for storage, cultivation and propagation of seagrass
Donor plants harvested from wild seagrass meadows, naturally uprooted vegetative
fragments or germinated seeds can be stored, cultivated and/or propagated in
nurseries before transplanting to restoration sites. Storage means the temporary
housing of seagrass plants until enough material is collected for planting or until
planting conditions are favourable (e.g., a season with high seagrass growth and/or
calm weather conditions). Plants can also be cultivated in a nursery until they reach a
larger size to increase their chances of post-transplant survival. Alternatively, adult
plants can be propagated to increase the amount of seagrass material for restoration.

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Nurseries can range from simple aquaria that fluctuate with the ambient
environmental conditions (e.g., Cade 2016; Figure 7) to sophisticated flow-through
systems with options to control environmental variables (e.g., light, temperature;
Zabarte-Maeztu et al. 2021). Although Z. muelleri fragments can be stored floating in
water before transplanting (Stafford-Bell et al. 2015; Cade 2016), planting seagrass
into a sand substrate is likely to be required for long-term cultivation. Sediment
composition may be important, with growth of nursery-cultivated Z. marina and P.
australis seedlings and fragments improved when sediments were finer (Tanner &
Parham 2010; Statton et al. 2013). When the sediment is not naturally rich in organic
matter, the addition of refractory organic matter (e.g., seagrass detritus) may also
enhance plant growth (Statton et al. 2013). However, the addition of seagrass detritus
to sediments that already contain organic matter (1.0-1.3% sediment dry weight) did
not improve the growth of Z. marina seedlings (Tanner & Parham 2010) and inhibited
growth of Halophila ovalis plants (Kilminster et al. 2006). Similarly the addition of
rapidly decomposing forms of organic matter (e.g., sucrose, fish feed and algae) could
have a negative effect on seagrass growth and survival (Statton et al. 2013).
Exogenous application of rooting hormones and fertiliser has been used to enhance
shoot production and rhizome growth (e.g., Fonseca et al. 1994; Sheridan et al. 1998;
Balestri & Bertini 2003; Tanner & Parham 2010) and could be trialled for nursery
cultivated Z. muelleri.
Figure 7. Growing seagrass (Zostera muelleri) in a simple nursery system in Aotearoa New
Zealand (Photos: Cade 2016).
Key considerations for nursery establishment include:
• seawater should be filtered
• seawater temperature remains within natural range for the local seagrass cultivar
• seawater salinity remains within natural range for the seagrass species
• sufficient light and nutrients are available for photosynthesis
• water remains oxygenated

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• algal growth is controlled
• suitable substrate is used if planting seagrass.
Several studies have demonstrated that it is possible to store or cultivate seagrass in
land-based nurseries (e.g., Balestri & Bertini 2003; Tanner & Parham 2010; Statton et
al. 2013; Cade 2016; Ferretto et al. 2021; Zabarte-Maeztu et al. 2021), in some cases
for as long as two to three years (e.g., Meinesz et al. 1991; Meinesz et al. 1993;
Balestri et al. 2011). For example, large-scale germination and culture of Z. marina
provided 15,000 seedlings that were planted out at a restoration site three months
after germination (Tanner & Parham 2010). Nursery-grown seedlings were found to
have higher survival rates, taller shoots and greater lateral expansion than plants
transplanted from wild seagrass meadows (Tanner & Parham 2010). Similarly,
Balestri et al. (2011) demonstrated that naturally uprooted P. oceanica seagrass
fragments cultivated in a nursery for three years maintained their capacity to grow and
re-establish when transplanted back into the wild.
One downside to this approach is that maintaining plants in a nursery could be
expensive. This cost, and the need to develop successful germination and cultivation
methods for many species, is likely the reason that seagrasses are not generally
commercially propagated for restoration (Shafer & Bergstrom 2008). The cost will
depend on the growing requirements of the seagrass species and will be higher where
electricity is required to power heat pumps, lights and flow-through water systems.
There are also large space requirements along with high labour costs associated with
planting and harvesting plants, frequent cleaning of the tanks and water changes.
Costs could be reduced if seagrass is able to be grown using a simple nursery system
(e.g., static tanks kept outdoors and exposed to ambient changes in light and
temperature) and maintained by community groups. The addition of invertebrate
grazers to tanks could be a cost-effective way to control excessive algal growth (e.g.,
Seddon et al. 2005).
2.3.3.
Transplanting seagrass plants
Once obtained, seagrass plants need to be transplanted to a restoration site. A variety
of transplantation techniques exist, ranging from simple direct planting to the more
complex use of stakes or structures to anchor and shelter the plants. Transplantation
of seagrass plants generally costs more than spreading or planting seeds because
there is more effort involved (Fishman et al. 2004; Tanner & Parham 2010).
Planting seagrass clods or plants in the seafloor
The least destructive form of transplantation is to plant seagrass clods or bare-rooted
plants directly into the seafloor sediments without the addition of stakes or other
artificial structures (Campbell 2002; Boudouresque et al. 2021). Transplantation has
been used for Zostera species worldwide using relatively small clods or bare-rooted
plants, with mixed success. It was successful in a Swedish transplantation study

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where survival and growth of unanchored Z. marina plants was higher than for those
that were anchored (Eriander et al. 2016). Conversely, in an Australian trial the
survival of transplanted Z. capricorni
9
was low and high costs were incurred along with
damage to the donor plots (McLennan & Sumpton 2005).
Paulo et al. (2019) identified that the size of the transplanted plot can influence
restoration success with only larger plots (up to 103 m2) of transplanted Z. marina
persisting over time along the Portuguese coastline. Larger scale (i.e., 43–1020 m2)
seagrass transplantation has also been trialled. Relatively large clods (e.g., ~ 1–3 m2
in area, 40–60 cm deep) of seagrass (none of these Zostera species) were
transplanted using balloons (Sánchez Lizaso et al. 2009), boat-cranes and barges
(Bedini et al. 2020), underwater harvesting and planting machines (Paling et al. 2001)
and mechanised boats and other equipment (Uhrin et al. 2009; Descamp et al. 2017),
with mixed success.
Fixing plants to the seafloor
Fixing seagrass plants to the seafloor using some form of anchoring system (e.g.,
stakes, staples, pegs, wire, nails) can help reduce the impacts of water motion (e.g.,
from storms and waves) on transplants thereby increasing survival success. Absence
of a sufficient anchoring system for transplants is a common reason for poor
restoration success (West et al. 1990; Bastyan & Cambridge 2008). Anchoring plants
can be relatively inexpensive, but it does require additional time to attach fixtures
(Fonseca et al. 1998). Various Zostera species have been successfully anchored to
the seafloor using staples (Fonseca et al. 1998), bamboo sticks (Davis & Short 1997),
wire anchors (West et al. 1990), oyster shells (Lee & Park 2008), stones (Zhou et al.
2014), and iron nails (Lange et al. in review, referenced in Tan et al. 2020). However,
depending on the hydrodynamic environment, anchors may still not be strong enough
to hold plants in position over the longer term (West et al. 1990). The use of anchoring
devices introduces artificial material into the environment (Fonseca et al. 1998), hence
natural or biodegradable options are desirable (e.g., bamboo stakes, stone or oyster
shells as described above, or the starch-based anchoring system described in
Scannavino et al. 2014).
Use of structures
In-water structures can improve seagrass restoration success by stabilising the
sediment and providing protection from water motion and herbivores (Tan et al. 2020;
Piazzi et al. 2021). The range of structures and material used for this purpose include
grids, mats, frames, and cages made of artificial (metal, concrete, plastic) or
biodegradable materials. Structures can be cost effective or relatively expensive,
depending on the technique used (Piazzi et al. 2021).
9
Synonymous with Zostera muelleri.

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Several structures have been trialled using Zostera species. For Z. muelleri, a study in
Australia demonstrated that frames to anchor seagrass plants, mats to protect from
bioturbating animals and cages to exclude herbivorous fish were the most successful
techniques of those investigated, although overall success of these techniques was
site dependent (Wendländer et al. 2020). Weighted wire frames that lie on the
seafloor and protect plants from bioturbation and uprooting have also been
successfully trialled for Z. marina in the USA (Short et al. 2002). There are various
examples for other seagrass species. For example, mats (degradable and non-
degradable) were used to enhance the survival of P. oceanica, minimise the loss of
transplanting plots and facilitate colonisation of other organisms (Piazzi et al. 2021)
and wire mesh cages were used to protect transplants (Cymodocea nodosa) from fish
grazing (Boudouresque et al. 2021). However, surrounding seagrass units with plastic
mesh did not improve plant survival in a high energy wave environment (van Keulen
et al. 2003). Concrete structures have also been used overseas in attempts to restore
P. oceanica, although this method is destructive to the environment given that the
plants may die but the cement could endure for centuries (Boudouresque et al. 2021).
Artificial seagrass that mimics the physical properties of living seagrass can also
improve survival of seagrass transplants. For example, artificial seagrass improved
hydrodynamic conditions for Z. marina by reducing wave height and current velocity
(Carus et al. 2020). For other seagrass species, this method has also been shown to
reduce damage to seagrass from herbivory (Tuya et al. 2017) and increase transplant
survival by stabilising sediment grain size (although not sediment erosion or accretion;
Campbell & Paling 2003).
Artificial structures can pollute the environment, and efforts have been made to
develop more sustainable options by using biodegradable materials (Tan et al. 2020).
Examples include hessian bags, mesh made from natural fibres, paper mesh and
coconut fiber pots (Gobert et al. 2005; Tanner 2015; Ward et al. 2020; Ferretto et al.
2021). Artificial seagrass made from biodegradable materials is also being developed
(SeaArt 2020).

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3. SEAGRASS RESTORATION IN AOTEAROA
3.1. Previous active restoration efforts
Active seagrass restoration has been trialled in four estuaries/harbours in Aotearoa
New Zealand: Manukau (Turner 1995), Whangarei (Matheson 2016; Matheson et al.
2017), Porirua (Zabarte-Maeztu 2021) and the Avon-Heathcote estuary (Gibson &
Marsden 2016). These studies relied on the transplantation of wild seagrass plants
and had varying degrees of success. The first restoration trial began in 1993 in
Manukau Harbour (Turner 1995), as an attempt to rehabilitate seagrass following
large losses between the 1960s and 80s (Inglis 2003). Seagrass was transplanted
both as intact sods and as bare-rooted plants anchored with staples. Unfortunately,
restoration was unsuccessful, with a decline in survival occurring after six months
following the onset of autumn storms (Turner 1995).
Like Manukau Harbour, Whangarei Harbour suffered extensive losses of seagrass in
the 1960s and 70s (Reed et al. 2004). Deteriorating water clarity and sediment
smothering caused by capital dredging operations and discharges from a cement
factory were likely the primary drivers of this loss (Reed et al. 2004). Seagrass
transplantation was trialled here in 2008 and 2012 following a feasibility study that
showed that water quality and sediment properties were suitable for restoration (Reed
et al. 2005). In the 2008 trial, three transplantation methods were tested: intact sods,
unanchored bare-rooted plants and bare-rooted plants planted within mats of artificial
seagrass (Matheson et al. 2017). Transplanting sods and bare-rooted plants was
equally effective, suggesting no benefit to keeping the root and rhizome system intact
and no negative effects of additional handling or lack of anchoring device using the
bare-rooted plant method. Conversely, planting within the mats was unsuccessful,
likely due to light limitation arising from the artificial canopy and associated biofouling,
and restrictions on macrofaunal bioturbation caused by the mesh base (Matheson et
al. 2017). A more than four-fold increase in seagrass extent was observed across the
1 ha site in the two years post-transplantation (Matheson et al. 2017). In 2012, four
planting configurations, ranging from a cluster of five small cores to larger plots of
0.5 m2, were tested at a second site in the harbour (Matheson 2016). All
configurations were equally successful four years post-transplant. Seagrass in
Whangarei Harbour has now increased by 600 ha (Tan et al. 2020) to reoccupy 40%
of its previous range (Matheson et al. 2017). However, it is likely that seagrass extent
would have increased without intervention following improvements in environmental
conditions (passive restoration, refer Section 2.1), making it difficult to assess the
contribution of active transplantation to this expansion. Nevertheless, it demonstrates
that it is possible to successfully transplant seagrass in Aotearoa New Zealand if the
environmental conditions are suitable.
In Porirua Harbour, seagrass extent has declined by 40% since 1980 (Matheson &
Wadhwa 2012). These losses have been linked to substrate muddiness and

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consequent unfavourable rhizosphere conditions as well as low irradiance and
smothering by sediment during storms (Zabarte-Maeztu et al. 2020; Zabarte-Maeztu
2021). Elevated water column nitrate, sediment contamination and smothering by sea
lettuce (Ulva spp.) and a filamentous green alga (Chaetomorpha ligustica) have also
been proposed as drivers of this change (Matheson & Wadhwa 2012; Zabarte-Maeztu
2021). In 2015 and 2019, two seagrass transplant trials were carried out in Porirua
Harbour (Zabarte-Maeztu et al. 2020; Zabarte-Maeztu 2021). Intact seagrass sods
transplanted on two occasions in 2015 did not survive more than 14 months (Tan et
al. 2020), likely due to low light availability, smothering by fine sediment and
unfavourable substrate conditions (Morrison 2021; Zabarte-Maeztu 2021). Seagrass
transplantation using intact sods and bare-rooted plants anchored with staples was
tested in 2019 (Zabarte-Maeztu 2021). After 45 days, seagrass retention ranged from
20–100%, with transplantation using sods the preferred method because seagrass
retention was high where environmental conditions were favourable, and it was easier
to relocate the transplanted seagrass. Zabarte-Maeztu (2021) found it difficult to
recognise the transplanted bare-rooted plants amongst the surrounding seagrass (in
control treatments) and challenging to stabilise them using staples in the dynamic
intertidal zone.
Extensive loss of seagrass occurred in Ihutai/Avon Heathcote estuary prior to 1929
with almost no seagrass remaining by 1952 (Inglis 2003). Since then, the extent of
seagrass bed has fluctuated and was reported to cover 52 km2 in 2016 (Gibson &
Marsden 2016). A transplant experiment was carried out in the estuary in 2016, using
intact sods harvested from two donor sites (Figure 8; Gibson & Marsden 2016).
However, after three months only four of the 15 transplanted sods were still alive. The
rest had either washed away, been smothered with sea lettuce (Ulva spp.) or died
after bleaching. Gibson and Marsden (2016) recommended that seagrass
transplantation is undertaken in the spring when seagrass growth has resumed.
Figure 8. Intact sods being transported (left) and in-situ at the restoration site (right) in the Avon-
Heathcote seagrass transplant experiment (Photos: Gibson & Marsden 2016).

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3.2. Information relevant to seed-based restoration potential
Seed-based restoration (refer Section 2.2) has not been trialled in Aotearoa New
Zealand because it was previously thought that sexual reproduction for Z. muelleri
occurred infrequently (Turner & Schwarz 2006). For example, extensive sampling
within Raglan Harbour found no evidence of a sediment seed bank (Cade 2016).
However, recent research has documented flowering by Z. muelleri (Figure 9) in
several locations across the country, indicating that seed-based methods may be an
option for restoration if viable seeds can be found.
Figure 9. Seagrass flowering in Zostera muelleri showing protruding stigma
10
to catch pollen for
fertilisation (Photo: Tracey Burton, from de Kock et al. 2016).
Seagrass flowering and/or seeding has now been documented in both the South
Island (Ramage & Schiel 1998; Ismail 2001; Zabarte-Maeztu 2021) and North Island
(Matheson et al. 2010; de Kock et al. 2016; Dos Santos & Matheson 2017). Flowers
were recently recorded in the Top of the South (Te Tau Ihu) from Golden Bay
(Tasman) to Ngakuta Bay in Queen Charlotte Sound, although no seeds were found
(pers comm., Zabarte-Maeztu, NIWA, 10 September 2021). Flowering has also been
observed at Kaikōura and Otago. The reproductive periodicity and output of seagrass
on two intertidal reefs in Kaikōura was studied over two years (Ramage & Schiel
1998). The number of flowering shoots was found to peak during March (late
summer), with no flowering shoots present during winter (July to September).
Flowering shoots were more abundant in the low intertidal zone in comparison to the
10
Part of a plant that is specially adapted to catch pollen

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middle and upper zones, as well as in seagrass patches associated with tidepools.
Seeds were found over summer (December to March), and appeared to germinate the
following spring (September). Ramage and Schiel (1999) also reported that seagrass
seedlings recruited into small areas of sediment in tidepools on intertidal reef
platforms in Kaikōura during spring (although few survived through summer due to
removal by wave action). Overall, they determined that successful recruitment by
seedlings may be rare or at least episodic at this location. A few flowering shoots were
also documented at an intertidal meadow in Otago Harbour during summer
(December and March) in 1997 and 1998. However, flowering was not followed in
detail during this study, with the data limited only to plants in experimental plots.
Germinating seagrass seedlings (on cockles in the intertidal) have also been
observed in Ihutai/Avon Heathcote estuary in Christchurch (Gibson & Marsden 2016).
In the North Island, the abundance of flowering shoots was quantified at four intertidal
sites in Tauranga Harbour every month over two and a half years by Dos Santos and
Matheson (2017). Low densities of flowering shoots were present annually in spring
and summer at some of the sites. Flowering shoots were associated with higher plant
cover and biomass, and larger leaf size. Dos Santos and Matheson (2017) looked for
seeds at one site but none were found, potentially due to the relatively low sampling
effort and/or low flowering density or the possibility that low genetic diversity in the
local seagrass population precluded successful fertilisation. Seagrass flowering was
also recorded monthly (November to January) at seven sites Raglan Harbour (de
Kock et al. 2016). Like the Tauranga study, flowers were associated with higher
seagrass cover and biomass and larger leaf size. They also found that flowers were
more common where in lower salinity areas and near seeps, streams and tide pools.
Despite the presence of flowers, no seeds were found. A single seagrass flowering
shoot was also discovered during a subtidal survey in the Bay of Islands (Matheson et
al. 2010).
Local iwi may also have mātauranga (pūrākau, tohu, whakapapa) relating to seagrass
flowering and seed production in their rohe (refer Section 4.2.4). Restoration
programmes should be sure to incorporate this information.
3.3. Potential for low impact forms of seagrass restoration
Up until now, seagrass restoration efforts in Aotearoa New Zealand have relied on
harvesting wild plants from donor sites and/or passive restoration through the
reduction or removal of stressors. Other methods of obtaining seagrass material for
restoration (i.e., collecting seeds or naturally uprooted seagrass fragments,
germinating seeds or propagating seagrass plants in nurseries) show potential but
have not yet been trialled here. The pros and cons of applying different restoration
methods in Aotearoa New Zealand are summarised in Table 1 and discussed below.

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3.3.1.
Passive restoration
Water quality is one of the key threats to seagrass in Aotearoa New Zealand (Turner
& Schwarz 2006), so the reduction of sediment and nutrient inputs to the coastal
environment provides opportunities for passive seagrass restoration. For example,
seagrass extent increased substantially within four years after wastewater diversion in
the Ihutai/Avon Heathcote estuary in Christchurch (Gibson & Marsden 2016; Zeldis et
al. 2020). Passive restoration leading to increased water clarity in Whangarei Harbour
was also critical to the success of active seagrass restoration in 2008 and 2012
(Matheson et al. 2017). Efforts to improve water quality through voluntary and
regulatory (e.g., National Policy Statement for Freshwater Management; NPS-FM
2014) actions are already occurring in Aotearoa New Zealand, but there is scope to
increase work in this space. In addition to improvements in water quality, there is also
opportunity to facilitate natural recovery on local scales by reducing physical impacts
such as those caused by vehicle and boating activities. Passive restoration to facilitate
natural recovery should be considered prior to active restoration, as the establishment
of appropriate environmental conditions is a prerequisite for successful active
restoration. However, natural recovery on large spatial scales can be slow; often
taking decades (Vaudrey et al. 2010; refer Section 4.4.2.). Active restoration offers the
potential to accelerate natural recovery rates.
3.3.2.
Seed-based restoration
Seed-based restoration relies on a source of seeds, ideally local. Although flowers
have now been observed in several seagrass meadows across Aotearoa New
Zealand, seeds have only been found in one location (Ramage & Schiel 1998). If a
suitable source of seeds could be found, seed-based restoration is likely to be feasible
in Aotearoa New Zealand, given the success of this approach for other species of
Zostera (e.g., Z. marina in the United States of America; Orth et al. 2020). To the best
of our knowledge, seed-based restoration has not yet been carried out for Z. muelleri.
However, the practicality of collecting, storing, and germinating Z. muelleri seeds was
recently assessed in Australia (Tan et al. 2020), providing useful guidance for
Aotearoa New Zealand. Spreading or planting seeds is a low cost, low effort and low
impact form of restoration.
3.3.3.
Obtaining plants for transplanting
Wild seagrass harvested from donor sites has been successfully transplanted in
Aotearoa New Zealand when environmental conditions were suitable for
establishment (e.g., Matheson et al. 2017). Although higher impact than the other
forms of restoration methods discussed, donor sites in Whangarei Harbour and Avon-
Heathcote estuary recovered relatively quickly (Gibson & Marsden 2016; Matheson et
al. 2017; refer Section 4.2.2 for more information). However, harvesting wild seagrass
for transplantation may not be a sustainable restoration method for large-scale
projects, or where donor meadows are small, fragmented or under pressure,

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especially given the conservation status of Z. muelleri (de Lange et al. 2018). It is
likely to become increasingly difficult to justify the removal of seagrass from donor
site, particularly if lower impact methods of restoration can be developed in Aotearoa
New Zealand.
The use of naturally uprooted seagrass fragments for seagrass restoration has not
been trialled in Aotearoa New Zealand but fragments of Z. muelleri (as beach-cast
wrack or floating in the water column) commonly occur adjacent to seagrass
meadows. Further work will be required to identify what environmental conditions are
most conducive to the generation of these fragments for a particular location. An
Aotearoa New Zealand study demonstrated that it is possible to store Z. muelleri
fragments for at least one month prior to transplanting, with neither the length of time
post-detachment nor the detachment season affecting rhizome or shoot growth six
weeks after planting in aquaria (Cade 2016). However, the authors noted that further
evidence is required to rule out a seasonal effect (i.e., fragments detached from plants
experiencing high growth rates and productivity during summer may have higher
survival rates; Cade 2016). We recommend trialling this method in Aotearoa New
Zealand because it offers a low impact alternative to harvesting wild plants and
community engagement could be used to reduce costs and foster engagement.
Germinating seeds and transplanting seedlings also places minimal pressure on wild
seagrass populations but requires more effort, and therefore has higher costs, than
spreading seeds directly at a restoration site. Although germination in cultured
conditions has not yet been demonstrated for Z. muelleri, it has been successful for
other Zostera species, indicating this restoration method may be possible in Aotearoa
New Zealand. Germination should be trialled if seeds can be found.
Propagating seagrass in a nursery offers the potential to provide a continuous supply
of seagrass plants for low-impact restoration. Although propagation has not yet been
trialled in Aotearoa New Zealand, several short-term experiments have demonstrated
that it is possible to grow Z. muelleri in a nursery for up to six weeks (Ramage &
Schiel 1998; Ismail 2001; Cade 2016; Zabarte-Maeztu et al. 2021). It is likely that
propagation of Z. muelleri in a nursery would be possible, given its success for Z.
marina (Tanner & Parham 2010). However, this method of seagrass restoration is
likely to be the most expensive because it requires long-term cultivation in a nursery.
3.3.4.
Transplanting
For plant-based active restoration, planting intact sods of seagrass is likely to be the
most successful method of transplantation in Aotearoa New Zealand. Although
Matheson et al. (2017) recorded equal success using unanchored bare-rooted plants,
Zabarte-Maeztu (2021) found it challenging to relocate bare-rooted plants and
stabilise them using staples. Transplantation of intact sods of seagrass is unlikely be
possible when seagrass material is obtained from germinated seeds, naturally

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uprooted vegetive fragments or propagated plants. If plants require anchoring or
shelter from environmental conditions or animals, then biodegradable or natural
materials should be used.

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Table 1. Summary of the pros and cons of different seagrass restoration methods for application in Aotearoa New Zealand.
Restoration method
Pros
Cons
Passive (facilitating natural recovery)
• Low impact
• Could be low cost (e.g., removing
localised impacts)
• Potential for wider ecosystem benefits
• Potential for wider human health benefits
• Could be slow
• Could be very expensive (e.g., catchment-scale
land management changes) with additional barriers
to overcome (e.g., societal, political)
• Unlikely in some situations (e.g., if seagrass loss
was due to reclamation)
Active
Seed-based
Collecting, processing and storing seeds
• Low impact
• Low cost
• Moderate effort
• Genetically diverse
• Untrialled in NZ
• Seeds have only been identified in one location in
NZ
• Usually requires a storage facility
Spreading/planting seeds
• Low impact
• Lower effort than planting clods or bare-
rooted plants
• Genetically diverse
• Untrialled in NZ
• Seeds have only been identified in one location in
NZ
Plant-based
Harvesting wild plants from donor sites
• Demonstrated success in NZ
• Higher impact than other methods
• High effort
• Low genetic diversity
• Unsustainable at large scales
Collecting naturally uprooted fragments
• Low impact
• Low effort
• Untrialled in NZ (but see Cade 2016)
• Source of material unreliable
• Low genetic diversity
• Requires storage facility

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Restoration method
Pros
Cons
Germinating seeds
• Low impact
• Supports genetic diversity
• Untrialled in NZ
• Seeds have only been identified in one location in
NZ
• High effort
• Requires a nursery
• Potential for high cost
Propagating of seagrass plants
• Low impact
• Potential to supply a continuous supply
of seagrass material
• Untrialled in NZ
• High effort
• Requires a nursery
• Potential for high cost
• Low genetic diversity
Transplantation
Planting clods or bare-rooted plants
• Demonstrated success in NZ
• Higher effort than spreading/planting seeds
Fixing plants to the seafloor
• More likely to withstand disturbance
• Addition of artificial material/litter into the
environment (unless biodegradable/natural
materials are used)
Use of structures
• More likely to withstand disturbance
• Addition of artificial material/litter into the
environment (unless biodegradable/natural
materials are used)
• Structures could damage seagrass

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4. DECISION MAKING FOR SEAGRASS RESTORATION
4.1. Decision making frameworks and considerations
To improve restoration success and to conduct restoration in an ethical manner, a
suite of decisions needs to be addressed prior to commencing restoration activities.
These decisions include identifying a suitable restoration location, determining what
restoration method is most likely to succeed, identifying a source of donor material,
and various environmental, ethical, financial and legal considerations. Although
gathering the information required to make these decisions may take substantial
effort, it is important for the reasons indicated above. Unfortunately, comprehensive
decision-making processes are often neglected in seagrass restoration
(Boudouresque et al. 2021).
Various frameworks have been developed to guide seagrass restoration decisions. In
Aotearoa New Zealand, a decision-making framework was created for a Z. muelleri
restoration trial in Whangarei Harbour (Schwarz et al. 2005). It included flow charts
with the necessary steps for (1) identifying a restoration site and requirements for
environmental enhancement, (2) conducting a trial, and (3) monitoring the success of
the trial, with other considerations discussed in the report text. Frameworks developed
overseas encompass additional aspects to the Schwarz et al. (2005) framework.
Despite being developed for other seagrass species, many of these frameworks could
be applicable to decision making in an Aotearoa New Zealand context bearing in mind
the biological differences between seagrass species where relevant. For example, a
comprehensive five-step program for was developed for P. australis restoration in
Western Australia (Campbell 2002). Planning processes were broken into five steps
(1) objective setting, (2) site selection, (3) transplant unit and technique, (4) habitat
enhancement and (5) review of objectives. Campbell (2002) also emphasises the
importance of cost:benefit analyses. Another example of decision-making frameworks
is the code of conduct and decision-making strategy for P. oceanica restoration
developed in France (Boudouresque et al. 2021). The code of conduct (Appendix 1)
considers factors such as whether there are wild meadows nearby, the reason for
restoration, experimental planting success, the risk to wild meadows and the larger
seagrass management strategy for the region. The decision-making strategy
(Appendix 2) outlines local- and regional-scale decisions relating to site selection as
well as consideration of investment trade-offs. Boudouresque et al. (2021) also
discuss in further detail ‘why’, ‘where’ and ‘when’ to conduct seagrass restoration.
Various overseas studies outline decision making considerations that are relevant to
seagrass restoration in Aotearoa New Zealand. These either build on information in
existing frameworks or outline topics not currently included in these frameworks.
Considerations relating to site conditions and restoration methods, such as the
importance of removing seagrass threats prior to replanting, proximity to (and
recovery of) donor meadows, site-specific planting techniques and size of the

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restoration effort size are discussed by van Katwijk et al. (2016). Seagrass genetics
are also important to consider, as seagrass populations may be adapted to local
condition, and therefore sourcing seagrass seeds or plants from a nearby population
(i.e., eco-sourcing) may be the best restoration approach (Jones et al. 2008). It is also
important to have enough genetic variation in seagrass donor material to avoid
genetic isolation and increase survivability under future conditions (van Katwijk et al.
2009; Tan et al. 2020). Future conditions at a restoration site are also important to
consider (Tan et al. 2020 and references within), given that restoration success
depends on whether seagrass (plants or seeds) can persist on a long-term basis
(refer Section 4.2.1 for seagrass environmental requirements). For example, future
impacts of climate change and other anthropogenic activities should be kept in mind
when making restoration-related decisions. It is also important to work with traditional
owners, as indigenous cultures have observed and managed natural environments
over long time periods and can give important insights, observations, and
interpretations relating to the state of these environments (Tan et al. 2020; refer
Section 4.2.4). Other people with ties to an area may also provide useful local
knowledge (Unsworth et al. 2019).
4.2. Information to support seagrass restoration decisions in Aotearoa
New Zealand
Below we provide information to support decision-making processes for seagrass
restoration in Aotearoa New Zealand. This includes information on the environmental
requirements, natural recovery timeframes, genetics of Z. muelleri and the role that iwi
could play in restoration initiatives.
4.2.1.
Key environmental requirements
An important precursor to successful seagrass restoration is the identification of a
suitable site for restoration. If seagrass does not currently exist in a particular location,
it is important to establish the reason for its absence. Where environmental conditions
are not suitable for the seagrass species, restoration is likely to fail. Simply relying on
the historical distribution of seagrass meadows is insufficient because even if the
source of stress has been removed, negative feedbacks may prevent the
reestablishment of suitable environmental conditions (e.g., Kemp et al. 2005; van Der
Heide et al. 2011). Below we outline current knowledge on the environmental
requirements of Z. muelleri plants in Aotearoa New Zealand (Table 2). These
environmental requirements can be used to check the suitability of a potential
restoration site or to model areas that could have a high chance of restoration
success (e.g., Davis et al. 2016; Kuusemäe et al. 2016; Vanglis 2016). In addition to
these environmental requirements, seagrass need to be free of physical impacts that
could cause damage to them. Environmental requirements of other stages of the

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lifecycle (e.g., seeds, seedlings) could differ; consideration of these in future work
could be useful to facilitate sexual reproduction.
Seagrass meadows occur along the length of Aotearoa New Zealand (Turner &
Schwarz 2006), suggesting that temperature is not a limiting factor for establishment.
They generally grow in the intertidal zone, but can be found at depths of at least 8 m
in some places (Clark & Crossett 2019). The upper depth distribution is determined by
the risk of seagrass drying out at low tide, photoinhibition due to high light irradiance,
wave exposure and associated turbidity, and reductions in salinity from freshwater
input (Turner & Schwarz 2006). The lower depth limit is primarily governed by light
availability (Hemminga & Duarte 2000). Light availability is a critical determinant of
whether seagrass will grow and persist at a site (Dennison et al. 1993). Studies in
Aotearoa New Zealand indicate that Z. muelleri requires at least 10–50 mol m-2 d-1 of
photosynthetically available radiation (PAR) or water clarity of more than 0.5 m
(Table 2). These high light requirements suggest restoration success will be more
likely in intertidal areas. High sedimentation rates can also reduce light availability as
well as cause direct mortality to seagrass via smothering (Sorensen 2020). Thus,
restoration attempts may not be successful where sedimentation rates are more than
2–3 mm yr-1 (Table 2).
Seagrasses use nutrients in the sediment porewater and the water column to support
their growth. Nutrient concentrations need to be sufficient for photosynthesis but not
so high that they stimulate algal growth, outcompeting the seagrass for light (Turner &
Schwarz 2006), or cause direct physiological effects that inhibit seagrass growth
(Burkholder et al. 1992; van Katwijk et al. 1997; Table 2). Robertson and Savage
(2020) suggest that once nutrient loadings surpass 50 mg N m-2 d-1, seagrass extent
will be severely reduced or absent. However, the resilience of Z. muelleri meadows to
nutrient enrichment is highly site-dependent (Gladstone-Gallagher et al. 2018). To the
best of our knowledge, nutrient thresholds have not been established for Z. muelleri.
However, water column nitrate concentrations of 80–170 µL-1 have been shown to
cause toxicity symptoms in a closely related species, Z. marina (Burkholder et al.
1994).
In Aotearoa New Zealand, Z. muelleri prefers muddy-sandy sediments that contain a
low level of organic matter (Table 2). However, seagrass will not grow where nutrient
porewater or hydrogen sulphide levels are too high (Table 2). Seagrasses can
bioaccumulate metals and other contaminants from both the water column and
sediment but limited information on toxicity thresholds is available for Z. muelleri
(Matheson & Wadhwa 2012; Zabarte-Maeztu 2021).
Seagrass meadows are commonly found in sheltered areas. This distribution is likely
controlled by the impacts that currents and waves have on meadow development and
spread (e.g., transportation of seeds and vegetative propagules, uprooting of
seedlings, and damage to mature plants) and the erosion, transport and deposition of

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sediment, nutrients or organic matter (Turner & Schwarz 2006). Previous restoration
attempts in Aotearoa New Zealand have failed due to storm damage (e.g., Turner
1995; Zabarte-Maeztu 2021) demonstrating the importance of choosing a sheltered
location for restoration, particularly in intertidal areas. Fonseca et al. (1998) caution
against attempting seagrass restoration in areas where current speeds exceed
0.5 m s-1.

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Table 2. Optimal environmental conditions for Zostera muelleri plants based on studies in Aotearoa New Zealand (modified from Schwarz et al. 2005; Zabarte-
Maeztu 2021). Seagrass also needs to be free of physical impacts that could cause damage.
Parameter
Too little
Suggested optimum
Too much
Source
Water depth
0.5–2.0 m
(Schwarz et al. 2005)
> 8 m
(Clark & Crossett 2019)
Tidal exposure (for intertidal meadows)
6 hours
2–5 hours
0-1 hour
(Schwarz et al. 2005)
Wave and current exposure
Currents speeds < 0.5 m s-
1*
(Fonseca et al. 1998)
Light (PAR)
25 mol m-2 d-1
(Matheson et al. 2017)
< 2.4 mol m-2 d-1
10-35 mol m-2 d-1
> 70 mol m-2 d-1
(Zabarte-Maeztu 2021)
< 2.1 mol m-2 d-1
10-50 mol m-2 d-1
Unlikely
(Bulmer et al. 2016)
Sedimentation rate
1-2 mm yr-1
> 3 mm yr-1
(Zabarte-Maeztu 2021)
Catchment nutrient loading
> 50 mg N m-2 d-1
(Robertson & Savage 2020)
Sediment mud content
8–23%
(Zabarte-Maeztu 2021)
< 13%
> 13%
(Park & Donald 1994)
< 5%
5–22%
> 23%
(Zabarte-Maeztu 2021)
Clay/silt
Silt/sand
Sand
(Schwarz et al. 2005)
Sediment organic matter
< 0.5%
0.5–3.0%
> 5%
(Zabarte-Maeztu 2021)
Sediment porewater nutrients
Ammonium < 30 µM
Ammonium > 70 µM
(Zabarte-Maeztu 2021)
Sediment porewater hydrogen sulphide
0–9.0 µM
10 µM
(Zabarte-Maeztu 2021)
Water clarity
< 0.5 m Secchi
> 0.5 m Secchi
(Schwarz et al. 2005)
Water column nutrients
Ammonium 3–62 µL-1
(Matheson et al. 2017)
Nitrate 0.5–12 µL-1
> 80 µL-1*
(Matheson et al. 2017), (Burkholder et al. 1994)
Phosphate 3–13 µL-1
(Matheson et al. 2017)
Temperature
Frosts
(Schwarz et al. 2005)
Salinity
15 ppt
29 ppt
36 ppt
(Matheson et al. 2017)
* Data from a study on Zostera marina carried out in the United States of America.

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4.2.2.
Timeframes for natural recovery
Understanding timeframes for natural seagrass recovery after a disturbance is
important for developing realistic restoration goals (Vaudrey et al. 2010; Cunha et al.
2012), as is understanding the trade-offs between passive and active restoration and,
evaluating the speed at which donor meadows will recover from harvesting. Seagrass
recovery timeframes depend on the type of disturbance and timeframe over which it
occurred (Turner & Schwarz 2006). Furthermore, recovery will only occur once
environmental conditions are suitable (refer Section 4.2.1). In some cases, the
potential for recolonisation is limited by dispersal capability (i.e., where there are no
wild seagrass meadows nearby; Turner & Schwarz 2006). Other factors that may
inhibit natural seagrass recovery include physical disturbance (e.g., from wind,
currents) and biological stressors (e.g., herbivore grazing; Wendländer et al. 2020).
Natural recovery can occur over relatively large spatial scales (e.g., populations or
meadows), for example, in association with catchment-level water quality
improvements. Timeframes for natural recovery of Zostera species over larger scales
(e.g., from a few to 1000s of hectares) are generally longer than five years and are
often more than a decade, although more rapid recovery has been documented at
smaller scales (e.g., 1000s of m2) (Vaudrey et al. 2010; Matheson et al. 2017). As
discussed in Section 3.1, it took more than 50 years before Z. muelleri meadows
began recovering from an historical increase (and then decrease) of fine sediment
inputs to Whangarei Harbour (Morrison 2021). Slowly improving conditions are
suspected to be the reason it took this length of time. However, Z. muelleri cover
increased relatively rapidly (from 1000 m2 to 4600 m2) over two years at a
transplanting site in Whangarei Harbour (Matheson et al. 2017). This expansion
largely took place over a relatively short period (from mid-spring to mid-summer).
While some of it could be attributed to active transplantation (up to 4.5 m2), much of it
occurred beyond the original transplant site (Matheson et al. 2017). Similar
timeframes for large-scale recovery of Z. marina have been reported, with areas
hectares in size taking more than five years (and often more than a decade) to
recover (Vaudrey et al. 2010), although recovery of a smaller area (0.8 ha) of Z.
marina occurred over three years (Vaudrey et al. 2010). In Tauranga Harbour
(Aotearoa New Zealand), damage (presumably at scales less than 1000 m in
distance; more likely to be 10s or 100s) to a seagrass bed caused by laying a pipeline
was evident two decades later (Park 1999).
At smaller spatial scales (i.e., < 0.25 m2), Zostera species are reported to have
relatively fast natural recovery due to high growth rates, and in some cases the
production of seed (Fonseca et al. 1998; Duarte et al. 2006). These scales are
relevant to very localised areas of physical disturbance, such as those caused by the
harvesting of plants (bare-rooted plants/clods) from donor plots. Zostera muelleri in
harvested donor plots from Whangarei Harbour (10 cm2, 15 cm deep) and Ihutai/Avon
Heathcote estuary (0.22 m2, 10 cm deep) recovered within nine and three months,

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respectively, likely through vegetative recolonisation (Matheson et al. 2017).
Relatively rapid recovery at small scales has been documented by various other
studies for both Z. muelleri (Peterken & Conacher 1997; Macreadie et al. 2014; Dos
Santos & Matheson 2017) and Z. marina (Rasheed 1999; Zhang et al. 2020).
4.2.3.
Seagrass genetics
Understanding genetic diversity and connectivity of seagrass populations is a critical
step in seagrass restoration. Knowledge of both dispersal mechanisms and seagrass
reproduction contributes to this understanding. For example, ocean currents can
disperse seagrass fragments and seeds over relatively long distances, while
vegetative growth expands from the existing seagrass meadow (Tan et al. 2020). In
Aotearoa New Zealand, Z. muelleri typically has restricted gene flow between
meadows (Sorensen 2020). Seagrass genetics differ between the North and South
Islands and between the East and West coasts, with these patterns found to be
associated with ocean currents (Jones et al. 2008). Some genetic separation was also
detected between sites within estuaries, although Z. muelleri was found to be
genetically similar at scales of metres and kilometres within these estuaries (Jones et
al. 2008). Diversity between seagrass populations in Aotearoa New Zealand is likely
related to dispersal of seagrass fragments rather than seeds (Cade 2016; Sorensen
2020). Fragments of Z. muelleri can be buoyant for a relatively long period, with the
timeframe potentially dependent on water temperature (Stafford-Bell et al. 2016;
Weatherall et al. 2016; refer Section 2.3.1). Hydrodynamic modelling tools could be
used to track where seagrass seeds and fragments might disperse to (Jackson et al.
2021)
Given that seagrass populations in Aotearoa New Zealand are genetically diverse, it
has been traditionally recommended that future restoration efforts use locally sourced
seagrass material (Jones et al. 2008). However, increasing the genetic diversity in a
population can also improve resilience, for example, to changing environmental
conditions associated with climate change (reviewed in Tan et al. 2020). It may be
beneficial therefore to include seagrass material from non-local (and potentially pre-
adapted) sources or mix seeds from multiple sources to improve fitness, particularly
for small populations with low genetic diversity. If Z. muelleri could be successfully
germinated or propagated in land-based nurseries there could also be opportunities to
selectively breed cultivars for certain traits (e.g., faster growth rates, greater tolerance
to environmental stressors or disease resilience). However, the ecological and ethical
implications of transplanting selectively bred plants into the wild would need to be
considered first.
Although not strictly genetics-related, biosecurity also should be considered if
seagrass material is being transferred between locations (e.g., the spread of
introduced plants/animals or potential pathogens). For example, the fungal wasting
disease Labyrinthula has been associated with declines in seagrass health in

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Aotearoa New Zealand (Morrison 2021), although it can also occur in healthy
seagrass meadows (Matheson et al. 2009; Clark & Crossett 2019).
4.2.4.
Iwi considerations
Seagrass decline, and the loss of whānau/hapū/iwi resources and mātauranga
(knowledge systems) associated with this habitat, is a particular issue for Māori
across Aotearoa New Zealand. These meadows provide habitat and mahinga kai
areas for taonga species and are a critical component of a healthy marine
environment. To maximise seagrass restoration success, initiatives should be co-
designed with mana whenua hapū/iwi to utilise their mātauranga and understand their
desires and priorities for restoration. Drawing on mātauranga-a-whānau/hapū/iwi of
historical seagrass distributions, and the values, uses, associations, and importance
of seagrass meadows will help guide restoration efforts. This knowledge, which often
extends further back in time than scientific observations, can be useful to counter
‘shifting baselines’, providing insight into what has been lost and aspirations for what
could be restored. Local whānau/hapū/iwi may have witnessed the stressors that led
to seagrass loss and could offer their view on whether these stressors have been
sufficiently reduced or removed to allow seagrass to recolonise an area.
Whānau/hapū/iwi may also hold mātauranga surrounding the flowering and seeding of
seagrass in Aotearoa New Zealand or views on the cultural appropriateness of
transferring seagrass plants between locations.
The Wai 262 claim relating to indigenous flora and fauna and Māori cultural
intellectual property (Waitangi Tribunal 2011) may have implications for the
propagation and transplantation of seagrass. The way that Wai 262 will be managed
by the New Zealand government is still evolving (Wheeler et al. 2021). It will be
important to develop a shared understanding of what this means for mana whenua in
restoration locations. For example, whānau/hapū/iwi may have views on the cultural
appropriateness of transferring seagrass plants between locations. Previous seagrass
restoration projects in Aotearoa New Zealand have been initiated by, carried out in
partnership with, and/or supported by, local iwi (e.g., Whangarei Harbour kaitiaki
roopu, Reed et al. 2004; Ngāti Toa in Porirua, Matheson & Wadhwa 2012).
4.3. Seagrass restoration potential for Whakatū/Nelson estuaries
There are four main estuaries within the Whakatū/Nelson region: Te Tai o
Aorere/Nelson Haven, Wakapuaka/Delaware Inlet, Waimeha/Waimea Inlet and
Kokorua
11
Inlet. Historical decline in seagrass has been recorded (or inferred) for all
these estuaries but the remaining seagrass meadows still cover relatively large areas
(except for Kokorua). Below, we review changes in the extent of these seagrass
meadows over time and the likely reasons for their decline. Based on this information
11
Called Whangamoa Inlet by Gillespie (2013).

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we provide a high-level assessment of the potential for seagrass restoration in these
estuaries. However, more detailed information (e.g., Reed et al. 2004; Reed et al.
2005; Matheson & Wadhwa 2012) is required to formally evaluate whether
environmental conditions (now and in the future) are suitable for seagrass restoration.
4.3.1.
Waimeha/Waimea Inlet
Waimea Inlet is a large (3,910 ha) estuary with a heavily modified catchment
dominated by exotic forest (33%) and pasture (20%; Stevens & Robertson 2017). This
estuary has lost more than 60% of its intertidal seagrass since records began in 1990,
with existing seagrass meadows now only covering 64 ha (2% of the intertidal area;
Stevens et al. 2020). However, Waimea Inlet was already extensively modified by
1990, therefore, the true extent seagrass of loss is likely much higher (e.g., 15% of
Nelson Haven is covered with seagrass). Most of this loss occurred prior to 1999 with
seagrass extent relatively stable since then (Stevens et al. 2020). Low coverage of
seagrass in Waimea is likely due to low water clarity forcing seagrass into areas of
higher tidal elevation that are vulnerable to desiccation stress at low tide (Stevens et
al. 2020). In addition, seagrass was lost following physical disturbance from the
installation of a sewage pipe in 2011, while other areas of seagrass are impacted by
fine sediment deposition (Stevens et al. 2020). The estuary is relatively muddy, largely
as a result of historical sediment inputs, and has high nutrient and contaminant inputs
in localised areas (Stevens & Robertson 2017; Stevens et al. 2020). As such, finding
an area with suitable environmental conditions for seagrass restoration could be
difficult.
4.3.2.
Te Tai o Aorere/Nelson Haven
Nelson Haven is a large (1,242 ha) estuary adjacent to Nelson City. Despite its urban
location, the catchment is dominated by native (54%) and exotic (33%) forest
(Stevens & Robertson 2017). Nelson Haven has lost 50% of its intertidal seagrass
since 1840. However, expansive seagrass meadows still exist, covering an area of
136 ha (15% of the intertidal area), and seagrass extent increased by 17 ha between
2009 and 2019 (Stevens & Forrest 2019b). Historical seagrass loss has been
attributed to reclamation (114 ha of seagrass was lost between 1931 and 1979 when
the port area was reclaimed) and changes in water quality (Gillespie et al. 2011a).
However, recent increases in seagrass extent, an improvement in the extent of muddy
sediments and a low eutrophication risk rating (Stevens & Forrest 2019b) suggest that
environmental conditions in this estuary are becomingly increasingly suitable for
seagrass colonisation. Active seagrass restoration could be used to accelerate this
process. However, this will need to be weighed up against the costs of carrying out
the restoration as expansion is likely occur through passive restoration alone. Nelson
Haven has relatively good water clarity because it has few large freshwater inflows,
and the main river has native forest in its upper catchment as well as a supply dam
that traps sediment (Zeldis et al. 2019). Despite this, the estuary still suffers from
elevated muddiness and local eutrophication and toxicity (Stevens & Robertson

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36
2017). Future land-use changes (e.g., forest harvesting, road development, urban
subdivision) could increase sediment loading to the estuary, making environmental
conditions less suitable for seagrass.
4.3.3.
Wakapuaka/Delaware Inlet
Delaware Inlet is a moderate-sized (335 ha) estuary that has been described as being
‘relatively pristine’ (Gillespie 2009), despite having an extensively modified catchment
(42% exotic forest; Stevens & Robertson 2017). Intertidal seagrass extent has
remained stable relative to 1983, covering 9.8 ha (2.5% of the intertidal area) of the
estuary (Stevens & Forrest 2019a). However, subtidal seagrass extent appears to
have declined since 1983 (Stevens & Forrest 2019a). There is some indication that
seagrass loss may have occurred between 1983 and 2009, as Gillespie et al. (2011b)
reported only 4.9 ha of seagrass in their survey (or 6.3 ha if patchier areas of
seagrass were included in the total). However, these changes could be due to
differences in survey methods (e.g., area mapped, or percent cover threshold required
for an area to be classified as seagrass). Loss of seagrass has been attributed to
increased muddiness near the Wakapuaka River Delta (Stevens & Forrest 2019a) as
well as physical damage from vehicles in localised areas (Gillespie et al. 2011b;
Šunde et al. 2017). Sedimentation is an issue for this estuary and forest harvesting in
the catchment may exacerbate this (Stevens & Forrest 2019a). Eutrophication and
toxicity also occur in localised areas (Stevens & Robertson 2017). Passive restoration
of seagrass meadows damaged by vehicles in Delaware Inlet has a high chance of
success if vehicle access is restricted or prohibited. Subtidal seagrass meadow
restoration is unlikely to be successful unless sedimentation is significantly reduced.
4.3.4.
Kokorua Inlet
Kokorua Inlet is a small (61 ha) estuary surrounded by exotic forest (45%), pasture
(4%) and native forest (51%; Stevens & Robertson 2017). A recent survey of this
estuary found no seagrass meadows (Stevens & Robertson 2015). Although seagrass
has not been recorded here historically, Gillespie (2013) reported in his preliminary
assessment of the environmental status of Kokorua Inlet that seagrass meadows
were expected to be present in limited areas within the estuary. Stevens and
Robertson (2015) suggest that the absence of seagrass is due to muddy sediments
and low clarity arising from sedimentation. A lack of seagrass propagules may be also
preventing seagrass reestablishment in parts of the estuary that are less affected by
sedimentation (i.e., mud/sand areas near the mouth of the estuary). As such, Kokorua
Inlet may benefit from active seagrass restoration efforts if areas exist with suitable
environmental conditions. However, it is uncertain whether seagrass was historically
present in this estuary and if so, the reason for its current absence. This information
should be established before attempting seagrass restoration.

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5. NEXT STEPS
Our review has identified several low impact seagrass restoration methods that could
be trialled in Aotearoa New Zealand. However, there are several areas that would
benefit from further investigation before applying these methods to a restoration
project. We have listed these in the context of seagrass restoration in the
Whakatū/Nelson region, but they could be applied elsewhere in Aotearoa New
Zealand. Steps to initiate decision making for seagrass restoration in the
Whakatū/Nelson region are also provided.
5.1. Trial of low impact seagrass restoration methods
• Seed-based active restoration:
o Quantitative survey of seagrass seeds and flowering within the
Whakatū/Nelson region.
o Engagement with local whānau/hapū/iwi to see if they have mātauranga
relating to seagrass flowering or seed production.
o If seeds can be identified, germination of seeds and growth of seedlings in
land-based nurseries and in situ within estuaries should be trialled.
o Evaluate the potential costs associated with seed-based restoration
approaches.
• Plant-based active restoration:
o Identify favourable conditions and locations for the collection of naturally
uprooted seagrass fragments in the Whakatū/Nelson region.
o Trial growing naturally uprooted seagrass fragments in a cost-effective
nursery system and compare their growth and survival to that of seagrass
plants harvested from wild meadows.
o Trial propagating seagrass plants in a cost-effective nursery system.
o Evaluate the potential costs associated with nursery-based restoration
approaches.
5.2. Steps to initiate decision making for seagrass restoration
• Develop a decision-making framework for seagrass restoration in the
Whakatū/Nelson region. Engagement with local whānau/hapū/iwi will be important
to ensure restoration guidelines are grounded in tikanga Māori and align with te ao
Māori values.
• Gather information underpinning decision-making for seagrass restoration in the
Whakatū/Nelson region:
o The historic and current distribution of seagrass meadows in the
Whakatū/Nelson region have been discussed in this report. Further
information (particularly for Kokorua Inlet, where no records of seagrass exist)
could be gathered by talking to local iwi and community members.

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o This report provides some information on impacts to local seagrass meadows
over time. Further information to determine whether the cause of seagrass
loss has been removed or reduced could be gathered by conducting an in-
depth review of available data as well as by talking to local iwi and community
members. Evidence of natural recolonisation of nearby seagrass beds also
indicates whether seagrass stressors have reduced in the general area.
o We also recommend conducting a quantitative survey of environmental
conditions at potential restoration sites to ensure they are suitable for
seagrass establishment.
o Collate or collect any other information required for decision making.
The decision-making framework for seagrass restoration in the Whakatū/Nelson
region can be used to specify the location and methods for seagrass restoration. It is
important to monitor the success of seagrass restoration, not only in terms of
seagrass expansion, but also with regard to ecological functioning (e.g., by using
proxies such as community composiiton, sediment grain-size and organic matter; see
Turner 1995).

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6. ACKNOWLEDGEMENTS
We thank Fleur Matheson and Iñigo Zabarte-Maeztu (NIWA) for a useful discussion
on flowering and seed production in Zostera muelleri in Aotearoa New Zealand as well
their experiences transplanting seagrass. Paul Gillespie (Cawthron) also provided
helpful advice in his review of this report.
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8. APPENDICES
Appendix 1. A code of conduct for seagrass (Posidonia oceanica) restoration. Figure
sourced from Boudouresque et al. (2021).

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Appendix 2. A decision-making strategy for transplanting Posidonia oceanica and other
seagrasses. The question–answer sequence first looks at the local level (the site
of the anticipated transplanting) and then the regional level (a homogenous area,
such as a bay). ‘No’ answers should lead to the project being abandoned. Figure
and caption text sourced from Boudouresque et al. (2021).