Eelgrass Growth Research Articles

Algal mats reduce eelgrass (Zostera marina L.) growth in mixed and monospecific meadows

Harmful algal blooms are considered to be one of the biggest threats to benthic vegetation. The amount of annual algal mats has increased due to eutrophication but their effects on perennial vegetation have rarely been tested in field experiments. In regions where eelgrass (Zostera marina) grows in mixed meadows, plant species richness may ameliorate algal-induced stress through facilitative mechanisms and improve eelgrass growth and survival. To test this, we conducted a 3mo. field experiment at 3.5m depth in the Baltic Sea, where we applied filamentous brown algae (300g ww) to eelgrass monocultures and polycultures (Z. marina, Ruppia cirrhosa, Stuckenia pectinata and Potamogeton perfoliatus). The algal stress period lasted for 13d and plots were sampled after a 5wk recovery period. In both algal-stressed mono- and polycultures, the presence of algal mats significantly reduced eelgrass shoot and root biomass production and affected the leaf growth negatively. Plant richness did not have strong effects on eelgrass growth and eelgrass in both algal-stressed mono- and polycultures showed significantly reduced productivity. Our results clearly demonstrate that loose-lying algal mats pose a serious threat to seagrass meadows and that plant diversity cannot alone ameliorate negative effects by macroalgal mats.

Spatiotemporal variability in the eelgrass Zostera marina L. in the north-eastern Baltic Sea: canopy structure and associated macrophyte and invertebrate communities

Seagrasses are marine angiosperms fulfilling important ecological functions in coastal ecosystems worldwide. Out of the 66 known seagrass species only two inhabit the Baltic Sea and only one, Zostera marina L., is found in its NE part. In the coastal waters of Estonia, where eelgrass grows at its salinity tolerance limit, only scarce information exists on the Z. marina community and there are no data on eelgrass growth. In the current study the community characteristics and growth of eelgrass were studied at four sites: Ahelaid, Saarnaki, and Soru in the West-Estonian Archipelago Sea and Prangli in the Gulf of Finland. Fieldwork was carried out from May to September in 2005. The results showed that eelgrass grew between 1.8 and 6 m with main distribution at 2-4 m. The eelgrass bed had a considerably higher content of sediment organic matter compared to the adjacent unvegetated areas, but this difference was statistically significant only in areas where the movement of soft sediments is higher. The results also showed that altogether 19 macrophytobenthic and 23 invertebrate taxa inhabited the eelgrass stand. The prevailing vascular plants were Stuckenia pectinata and Potamogeton perfoliatus. Besides attached macrophytes, drifting algae were recorded within the eelgrass communities throughout the study period. Most common invertebrate species inhabiting eelgrass stands were Peringia ulvae, Cerastoderma glaucum, Mytilus trossulus, Macoma balthica, Mya arenaria, Theodoxus fluviatilis, and Idotea chelipes. Irrespective of the studied basin, the increasing eelgrass density supported an elevated diversity and abundance of benthic invertebrates. The eelgrass shoot biomass showed a constant increase from May to September. The overall growth pattern was similar for all studied areas but varied among depth strata. The average shoot density of eelgrass was 50-1300 shoot per m 2 , average biomass ranged from (0.75) 7.8 to 37.31 g dw m -2 in spring and from 18.42 to 68.59 g dw m -2 in autumn.

Dependence of eelgrass (Zostera marina) light requirements on sediment organic matter in Massachusetts coastal bays: Implications for remediation and restoration

Using a calibrated bio-optical model we determined that the optical water quality conditions in several nitrogen-impaired embayments and in one unimpaired system were within the range of values known to support eelgrass growth. We also used the model to identify a range of light requirements for eelgrass (Zostera marina). Higher eelgrass light requirements, expressed as a percentage of surface-incident irradiance, corresponded with higher sediment organic matter content. These results corroborated findings by previous studies which indicate a generalized relationship: seagrasses growing in turbid conditions with poorer water and sediment quality have higher light requirements than those growing in less degraded conditions. The mechanistic reason for the variation in light requirements is still not completely explained and cannot be attributed to a single independent variable. Varying light requirement have important implications for eelgrass protection and should be considered when setting restoration targets for eelgrass in water quality and nitrogen remediation programs.

Nitrogen budget of the eelgrass, Zostera marina in a bay system on the south coast of Korea

Above- and below-ground productivities and tissue N content were measured monthly to quantify N incorporation to sustain eelgrass growth in Koje Bay on the south coast of Korea from January to December 2002. N acquisition was also estimated through measurements of N uptake kinetics, tissue biomass, and in situ inorganic N concentrations in water column and sediments. Above- and below-ground productivities were highest in summer and lowest in late fall and winter. Leaf tissue N content was highest in December and lowest in July, while rhizome tissue N content was highest in October and lowest in April. Estimated monthly N incorporation by leaf tissues based on the leaf productivity and N content ranged from 0.4 g N m−2 month−1 in November to 2.0 g N m−2 month−1 in May. N incorporation by below-ground tissues ranged from 0.1 g N m−2 month−1 in February to 0.2 g N m−2 month−1 in October. Annual whole plant N incorporation was 14.5 g N m−2 y−1, and N incorporation by leaf tissues accounted for about 87 % of total N incorporation. Maximum uptake rate (Vmax) and half saturation constant (Km) of leaf NH4+ uptake were significantly lower than those of root NH4+ uptake. Above- and below-ground biomass ranged from 20.8 g DW m−2 and 8.6 g DW m−2 in winter to 350.0 g DW m−2 and 81.3 g DW m−2 in spring, respectively. NH4+ concentrations varied from 0.2 to 4.3 mM in water column and from 93.0 to 551.7 mM in sediment pore water. Based on these measurements, annual N acquisition by root tissues contributed slightly higher than that by leaf tissues to total plant N acquisition. During winter, monthly leaf N acquisition was lower than monthly leaf N incorporation. This implies that Z. marina has internal nitrogen retention system to offset the shortage and excess of nitrogen.

Using allometric procedures to substantiate the plastochrone method for eelgrass leaf growth assessments

Estimation of leaf productivity in eelgrass (Zostera marina L.) is crucial for evaluating the ecological role of this important seagrass species. Although leaf marking techniques are widely used to obtain estimates of leaf productivity, the accuracy of these assessments, has been questioned mainly because these fail to account for leaf growth bellow the reference mark and also because they apparently disregard the contribution of mature leaf tissues to the growth rate of leaves. On the other hand, the plastochrone method is a simpler technique that has been considered to effectively capture growth in a more realistic way, thereby providing more accurate assessments of both above- and below-ground productivities. But since the actual values of eelgrass growth rates are difficult to obtain, the worth of the plastochrone method has been largely vindicated because it produces assessments that overestimate productivity as compared to estimates obtained by leaf marking. Additionally, whenever eelgrass leaf biomass can be allometrically scaled in terms of matching leaf length in a consistent way, the associated leaf growth rates can be also projected allometrically. In this contribution, we used that approach to derive an authentication of the plastochrone method and formally demonstrate that, as has been claimed to occur for leaf marking approaches, the plastochrone method itself underestimates actual values of eelgrass leaf growth rates. We also show that this unavoidable bias is mainly due to the inadequacy of single-leaf biomass assessments in providing a proxy for the growth of all leaf tissue in a shoot over a given interval. Moreover, the derived formulae give conditions under which assessments of leaf growth rates using the plastochrone method would systematically underestimate matching values obtained by leaf marking procedures. And, assessments of leaf growth rates obtained by using the present data show that plastochrone method estimations underestimated corresponding proxies obtained allometrically (27%), or through leaf marking (35%). Allometric projection is recommended as a simpler and more effective procedure to reduce the bias in eelgrass leaf productivity estimations that associates to the use of plastochrone methods.

Seasonal effects of clams(Panopea generosa)on eelgrass(Zostera marina)density but not recovery dynamics at an intertidal site

Today's marine ecosystems experience multiple stressors that complicate management and pose risks to resource protection. Clam aquaculture has several potential environmental effects including disturbance, space occupation, and modification of nutrient availability, which were assessed through a crossed experimental design of eelgrass (Zostera marina L.) removal and geoduck clam (Panopea generosa Gould) addition in 1 m2 plots, in an eelgrass meadow in Puget Sound, Washington, USA. Gaps recovered via clonal growth, with gap edges requiring more than 1 year and gap centres almost 2 years. Clams neither increased nor impeded eelgrass recovery, and clams did not reduce winter density of eelgrass. During summer, clams reduced eelgrass density by 30% but increased shoot length by 13% and clonal branching by 9%. Eelgrass growth rates (~1 mgDW shoot-1 d-1) and C:N ratios (~11) did not differ across clam-addition treatments, indicating no enhancement by fertilization. At the end of the 2-year experiment, clams were harvested, and this disturbance reduced eelgrass density by 70% in the geoduck plots. As managers consider expansion of intertidal geoduck aquaculture regionally, these data indicate only a minor effect on eelgrass from the presence of clams themselves, but harvest disturbance is comparable with other studies of fishing in eelgrass. Nets and tubes that serve as predator-exclusion devices during the early stages of the 5-year crop cycle for geoduck clams remain to be examined for their ecological effects. Copyright © 2012 John Wiley & Sons, Ltd.

Wave and tidally driven flows in eelgrass beds and their effect on sediment suspension

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 448:271-287 (2012) - DOI: https://doi.org/10.3354/meps09225 Wave and tidally driven flows in eelgrass beds and their effect on sediment suspension Jennifer C. R. Hansen, Matthew A. Reidenbach* Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22904, USA *Email: reidenbach@virginia.edu ABSTRACT: Seagrass beds alter their hydrodynamic environment by inducing drag on the flow, thereby attenuating wave energy and near-bottom currents. This alters the turbulent structure and shear stresses within and around the seagrass bed that are responsible for the suspension and deposition of sediment. To quantify these interactions, velocity, pressure, and sediment measurements were obtained across a density gradient of an eelgrass Zostera marina bed within a shallow coastal bay (1 to 2 m depth). Eelgrass beds were found to reduce near-bottom mean velocities by 70 to 90%, while wave heights were reduced 45 to 70% compared to an adjacent unvegetated region. Wave orbital velocities within the eelgrass bed were reduced by 20% compared to flow above the bed, primarily acting as a low-pass filter by removing high-frequency wave motion. However, relatively little reduction in wave energy occurred at lower wave frequencies, suggesting that longer period waves were able to effectively penetrate the seagrass meadow. Average bottom shear stresses (τb) at the unvegetated region were τb = 0.17 ± 0.08 N m–2, significantly larger than the critical stress threshold necessary for sediment entrainment of 0.04 N m–2. Within the eelgrass bed, τb = 0.03 ± 0.02 N m–2 and stresses were below the critical stress threshold during 80% of the time period of measurement. Expansion of eelgrass within the coastal bay has thus altered the dynamics of the seafloor from an erosional environment to one that promotes deposition of suspended sediment, enhancing light penetration throughout the water column and creating a positive feedback for eelgrass growth. KEY WORDS: Zostera marina · Seagrass · Waves · Turbulence · Boundary layer · Sediment resuspension Full text in pdf format PreviousNextCite this article as: Hansen JCR, Reidenbach MA (2012) Wave and tidally driven flows in eelgrass beds and their effect on sediment suspension. Mar Ecol Prog Ser 448:271-287. https://doi.org/10.3354/meps09225 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 448. Online publication date: February 23, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Effects of invasive colonial tunicates and a native sponge on the growth, survival, and light attenuation of eelgrass (Zostera marina)

We examined the effects of invasive colonial tunicates (golden star, Botryllus schlosseri; violet Botrylloides violaceus), and the native breadcrumb sponge (Halichondria panicea) on the growth, survival, and light attenuation of eelgrass (Zostera marina). Eelgrass shoot growth and survival were higher for unfouled shoots than for fouled shoots, and dependent on fouling species identity. Growth was lowest for shoots with violet tunicate fouling, and survival was lowest for shoots with sponge fouling. A large proportion (0.20-0.38) of fouled shoots marked for growth measurements were not found during retrieval compared to unfouled plants (0.08), suggesting that fouling led to premature breaking away of shoots and blades. Transmission of incident photosynthetically active radiation (PAR) through fouled blades decreased exponentially with increasing biomass of most fouling species. Relatively low biomass (1-1.5 dry mg cm -2 of blade) of the fouling organisms reduced light transmission by up to 98 % compared to unfouled blades. The proportion of incident PAR attenuated by the sponge and orange and burgundy morphs of the violet tunicate increased hyperbolically across their biomass, and reached a plateau at ~0.5 dry mg cm-2 of blade. The golden star tunicate and cream morph of the violet tunicate attenuated incident PAR linearly across their biomass. For the range of biomass examined, all fouling species attenuated 65-95 % of incident PAR prior to it reaching the blade. The reduction in light transmission was likely the causal mechanism underlying reduced growth of fouled shoots. Fouling of eelgrass by the invasive colonial tunicates and the native sponge will have numerous ecological consequences, including reduced productivity and coverage of eelgrass beds.

The impact of the herbicide atrazine on growth and photosynthesis of seagrass, Zostera marina (L.), seedlings

The impact of the widely used herbicide atrazine on seedling growth and photosynthesis of eelgrass was determined. The long-term impact of the herbicide atrazine (1, 10 and 100μg/L) on growth of eelgrass Zostera marina (L.) seedlings, maintained in outdoor aquaria, was monitored over 4weeks. Exposure to 10μg/L atrazine resulted in significantly lower plant fresh weight and total chlorophyll concentration and up to 86.67% mortality at the 100μg/L concentration. Short-term photosynthetic stress on eelgrass seedlings was determined and compared with adult eelgrass using chlorophyll fluorescence. The effective quantum yield in eelgrass seedlings was significantly depressed at all atrazine concentrations (2, 4, 8, 16, 32 and 64μg/L) even within 2h and remained at a lower level than for adult plants for each concentration. These results indicate that atrazine presents a potential threat to seagrass seedling functioning and that the impact is much higher than for adult plants.

The Growth of Estuarine Resources (Zostera marina, Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians, Cyprinodon variegatus) in Response to Nutrient Loading and Enhanced Suspension Feeding by Adult Shellfish

While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspension-feeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.

Study on Evaluation of Growth Factors for Eelgrass in the Semi-Enclosed Bay

Eelgrass beds have important functions as water purification, spawning and nursery grounds for valuable marine resources. In recent several decades, eelgrass beds have been decreased by coastal reclamation. In order to suppress that phenomenon, various environments improvement technology has been tested. However, the long-term conservation of constructed eelgrass bed construction is difficult and there are not many success cases. It is considered that it is attributed to the surrounding condition in the objective marine area is not suitable for the optimum eelgrass habitat environment. In this study, we conducted the evaluation and analysis of eelgrass growth factors using the Self Organization Future Map (SOM) that kinds of neural network. Also, we verify the complicated relationship between the eelgrass growth factor and changes of marine environment. It would be the effective approach for the sea grass bed construction.

High mortality of Zostera marina under high temperature regimes but minor effects of the invasive macroalgae Gracilaria vermiculophylla

The present study tested for density-dependent effects of the invasive drift macroalgae Gracilaria vermiculophylla (Ohmi) Papenfuss on growth and survival of the native eelgrass, Zostera marina L., under different temperature levels. Three weeks laboratory experiments were conducted in Odense, Denmark, combining three algae densities (control, low 1.9 kg WW m −2, high 4.5 kg WW m −2) with typical Danish summer temperatures (18 °C) and elevated temperatures (21 °C and 27 °C). There was a significant effect of temperature on shoot survival with on average 68% mortality in the high temperature treatment but almost no mortality at the two lower temperatures. The higher mortality was probably caused by high sulphide levels in the sediment pore water (0.6 mmol l −1 at 18 °C compared to 3.7 mmol l −1 at 27 °C). Above-ground growth of the surviving shoots was also significantly affected by temperature, with leaf elongation rates being negatively affected, while the leaf plastochrone interval increased. Relative growth rate was significantly higher at 21 °C than at 18 °C or 27 °C, whereas rhizome elongation was significantly lowest at 27 °C. Elemental sulphur content in the plant tissues increased significantly with temperature and was up to 34 times higher (S 0 in rhizomes) at 27 °C compared to the lower temperatures. In contrast to the temperature effects, cover by G. vermiculophylla did not cause significant effects on any seagrass responses. However, there was a (non-significant) negative effect of algal cover at the highest temperature, where the seagrass is already stressed. The latter results suggest that more studies should test for interaction effects between temperature and other anthropogenic stressors given that temperature is predicted to increase in the near future.

Spatial patterns of epifaunal communities in San Francisco Bay eelgrass (Zostera marina) beds

AbstractEpifaunal invertebrate species, such as amphipods and isopods, have been shown to play key but varying roles in the functioning of seagrass habitats. In this study, we characterized patterns in the poorly known epifaunal communities in eelgrass (Zostera marina) beds in San Francisco Bay as a first step in understanding the individual and collective importance of these species, while testing predictions on spatial patterns derived from previous studies in other regions. Surveys conducted at five beds across multiple time periods (April, June, August and October 2007) showed that San Francisco Bay eelgrass beds varied strongly in epifaunal community composition, total, and relative abundance, and that abundance differed markedly among time periods. In contrast to findings by others, morphologically complex flowering shoots frequently harbored greater numbers of epifauna (>2× and up to 10× more individuals) than vegetative shoots, but not different species assemblages. Similar to previous studies, several abiotic factors did not explain patterns in distribution and abundance among beds. The proportion of introduced species was very high (>90% of all individuals), a finding unique among seagrass epifaunal studies to date. Defining numerical patterns in epifaunal communities will inform related efforts to understand effects of epifaunal species and assemblages on eelgrass growth dynamics, seed production, and higher order trophic interactions over space and time.

A model for measuring leaf growth rate of vegetative shoot in the seagrass, Zostera marina

A new model for determining leaf growth in vegetative shoots of the seagrass Zostera marina (eelgrass) is described. This model requires the weights of individual mature and immature whole leaves and leaf plastochrone interval ( P L) as parameters, differing from the conventional leaf marking technique (CLM) that requires cutting and separation between new and old tissue of leaves. The techniques required for the model are the same as for the plastochrone method, but the parameters differ between both methods in use of the weight of individual immature leaves. In a mesocosm study, eelgrass growth was examined, and parameters for the new model and plastochrone method (the weights of individual mature and immature leaves and P L) were measured. Leaf growth rate was measured using the CLM and determined by the new method and the plastochrone method. The results were then compared between the CLM, the new model, and the plastochrone method. The results obtained with the new model were similar to those obtained with the CLM. However, the results of the plastochrone method differed from those of the CLM, while the weight of immature leaves varied seasonally. The new model was also used to determine leaf growth in a natural eelgrass bed in Mikawa Bay, Japan, and revealed the growth rates in all shoots and those of different ages. This method would be advantageous as an accurate means of direct measurement in fieldwork, and should therefore be a useful tool for monitoring seagrass growth.

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations.

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m−2) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).

Relative importance of trophic interactions and nutrient enrichment in seagrass ecosystems: A broad‐scale field experiment in the Baltic−Skagerrak area

The interaction of eutrophication and predation in structuring seagrass Zostera marina L. ecosystems was assessed in a field experiment in three regions along an estuarine salinity gradient, from southern Finland to the Skagerrak area of the Swedish west coast. All regions are considered to be affected by eutrophication and overfishing but differ in the abundance of intermediate predators (e.g., small fish, shrimp, and crabs), mesograzers, and the biomass of epiphytic algae. Using transplanted Zostera (eelgrass), nutrient levels and intermediate predator abundance were manipulated in a full‐factorial cage experiment. On the Swedish west coast, where ambient densities of mesograzers are very low, epiphytic algae responded strongly to nutrient enrichment, resulting in significantly reduced growth of eelgrass. At the Baltic sites however, where ambient densities of mesograzers are high, no significant growth of epiphytic algae was detected, and only grazer biomass responded to nutrient enrichment. Predation from small fish, shrimp, and crabs decreased the biomass of mesograzers by > 98% on the Swedish west coast, but natural predators had no significant effect on mesograzers biomass at the Baltic sites. Predation and nutrient enrichment interacted to affect the growth of eelgrass by controlling the biomass of mesograzers and nuisance algae. The differing effect of nutrient enrichment and grazing in the three regions may therefore be a result of the prevailing low and high predation pressure on mesograzers in Zostera. This absence or presence of predation may derive from interregional changes in trophic interactions, possibly caused by a combination of eutrophication and overfishing.

Successful Eelgrass (Zostera marina) Restoration in a Formerly Eutrophic Estuary (Boston Harbor) Supports the Use of a Multifaceted Watershed Approach to Mitigating Eelgrass Loss

From a watershed perspective, Boston Harbor, MA, USA is an ideal site for eelgrass restoration due to major wastewater improvements. Therefore, by focusing on site selection and transplant methods, high survival and expansion rates were recorded at four large eelgrass-restoration sites planted in Boston Harbor as partial mitigation for a pipeline construction project. Transplanted sites met and exceeded reference and donor bed habitat function after 2 years. Hand planting and seeding in checkerboard-patterned transplant plots were efficient and effective methods for jump-starting eelgrass growth over large areas. Although restoration through planting can be successful, it is highly site specific. Even using a published site-selection model, intensive fieldwork was required to identify sites at fine enough scale to ensure successful planting. Given the effort required to identify scarce potential sites, we recommend that future focus includes alternative mitigation strategies that can more adequately prevent eelgrass loss and address water quality degradation which is the leading cause of dieback, site unsuitability for planting, and lack of natural re-colonization.

Photosynthetic and morphological responses of eelgrass ( Zostera marina L.) to a gradient of light conditions

Zostera marina L. (eelgrass) from Great Bay Estuary, New Hampshire and Maine (USA), was transplanted in outdoor mesocosms and subjected to four light treatments (100, 58, 34 and 11% surface irradiance, SI) between May and September 2003 to investigate the relationship between light availability and the growth and survival of eelgrass. Evaluating eelgrass seedlings and adult mature plants demonstrated no differences in photosynthetic response after 22 days of acclimation. During at least the first 19 days of shading, maximum electron transport rate (ETR max) rate of eelgrass did not differ significantly between light treatments. After 40 days, a significant reduction in ETR max and minimum saturating light was observed in plants growing at 34% SI and below. Morphological responses exhibited a linear increasing trend with greater light. 34% SI exhibited drastic reductions (to less than 25% of control) in rhizome growth, shoot density, shoot production, number of nodes per plant and plant weight at the end of the study (81 days). Shoot to root ratio at 34% SI increased by > 50%. Plants shaded to 58% SI showed no significant difference from the control in plant parameters except an increased rate of rhizome elongation. Our results link the lower shoot densities with shading to the slow growth rate of horizontal rhizomes and a total lack of lateral expansion at 11% SI. ETR max declined over time in plants at 11% SI resulting in 81% mortality, no lateral branching and no morphological development, indicating that the minimum light required for long-term eelgrass growth and survival is greater than the previously suggested 11% SI. We demonstrate that eelgrass plants at these latitudes can persist at light levels of 58% SI and above, and are light-limited at 34% SI and below.

Effects of Oyster Depuration Gear on Eelgrass (Zostera marinaL.) in a Low Density Aquaculture Site in Long Island Sound

Oyster (Crassostrea virginica) aquaculture has a long history and tradition in Long Island Sound (Connecticut, USA). Although most of the producers practice traditional on-bottom aquaculture, there are a growing number of individuals utilizing bottom gear for cultivation and depuration. The use of this gear presents a potential conflict in eastern Long Island Sound where the last remaining populations of eelgrass (Zostera marina L.) exist. Shellfish aquaculture activity has been identified as a potential source for negative impacts to eelgrass populations. However, bivalve aquaculture has also been shown to provide an equivalent or greater degree of ecosystem services as submerged aquatic vegetation. The effects of short-term oyster depuration activity were gauged by comparing eelgrass reference sites and experimental plots (eelgrass areas containing oyster depuration cages with and without oysters) in triplicate. Changes in sheath length of the eelgrass 1 m from the cages were used as a proxy for growth rate. The aquaculture gear had no effect on this measure of growth rate of eelgrass in any of the deployments. Sediment characteristics (sediment chlorophyll, sediment % organics) in the cage footprint and 1m from the cages also failed to show an effect of the depuration cages on the local environment. Video monitoring of the footprints and local area indicated little physical damage to the eelgrass beds as a result of the short deployment of the aquaculture gear. The water column at all three sites was vertically well mixed and no effect of the cages on water column light and other characteristics was detectable. The results of this study indicated that at the current level of activity, short-term depuration of oysters has minimal effect on eelgrass growth, water quality and the sediment characteristics measured. However, if depuration activity expands in terms of the amount of gear and/or individual operations, it may result in measurable effects. Understanding the interactions between shellfish aquaculture activity and the marine environment is necessary for sustainable growth of the industry.