Eelgrass Beds Research Articles

Disturbance of eelgrass Zostera marina by commercial mussel Mytilus edulis harvesting in Maine: dragging impacts and habitat recovery

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 285:57-73 (2005) - doi:10.3354/meps285057 Disturbance of eelgrass Zostera marina by commercial mussel Mytilus edulis harvesting in Maine: dragging impacts and habitat recovery Hilary A. Neckles1,*, Frederick T. Short2, Seth Barker3, Blaine S. Kopp1 1USGS Patuxent Wildlife Research Center, 196 Whitten Road, Augusta, Maine 04330, USA 2Jackson Estuarine Laboratory, University of New Hampshire, 85 Adams Point Road, Durham, New Hampshire 03824, USA 3Maine Department of Marine Resources, PO Box 8, West Boothbay Harbor, Maine 04575, USA *Email: hilary_neckles@usgs.gov ABSTRACT: We studied the effects of commercial harvest of blue mussels Mytilus edulis on eelgrass Zostera marina L. in Maquoit Bay, Maine, USA, at a hierarchy of scales. We used aerial photography, underwater video, and eelgrass population- and shoot-based measurements to quantify dragging impacts within 4 sites that had been disturbed at different times over an approximate 7 yr interval, and to project eelgrass meadow recovery rates. Dragging had disturbed 10% of the eelgrass cover in Maquoit Bay, with dragged sites ranging from 3.4 to 31.8 ha in size. Dragging removed above- and belowground plant material from the majority of the bottom in the disturbed sites. One year following dragging, eelgrass shoot density, shoot height and total biomass of disturbed sites averaged respectively 2 to 3%, 46 to 61% and <1% that of the reference sites. Substantial differences in eelgrass biomass persisted between disturbed and reference sites up to 7 yr after dragging. Dragging did not affect physical characteristics of the sediment. The pattern and rate of eelgrass bed recovery depended strongly on initial dragging intensity; areas of relatively light dragging with many remnant eelgrass patches (i.e. patches that were missed by the mussel dredge) showed considerable revegetation in 1 yr. However, by developing recovery trajectories from measurements at sites disturbed in different years, we projected that it would require a mean of 10.6 yr for recovery of eelgrass shoot density within the areas of intense dragging characterizing most of the disturbed sites. A spatial simulation model based on measured rates of lateral patch-expansion (mean 12.5 cm yr-1) and new-patch recruitment (mean 0.19 patches m-2 yr-1) yielded a mean bed recovery time of 9 to 11 yr following dragging, depending on initial degree of plant removal. Model simulations suggested that with favorable environmental conditions, eelgrass beds might recover from dragging disturbance in 6 yr; conversely, recovery under conditions less conducive to eelgrass growth could require 20 yr or longer. This study shows that mussel dragging poses a severe threat to eelgrass in this region and that regulations to protect eelgrass from dragging impacts would maintain the integrity of a substantial amount of habitat. KEY WORDS: Eelgrass · Zostera marina · Seagrass · Dragging impacts · Recovery · Patch recruitment · Patch expansion Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 285. Online publication date: January 19, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Growth of an Artificial Eelgrass Bed on the Coast of Hinase-cho, Okayama Prefecture

Growth of an Artificial Eelgrass Bed on the Coast of Hinase-cho, Okayama Prefecture

葉上浮泥による光量低下と砂面変動がアマモ場の分布に及ぼす影響―広島湾でのケーススタディ―

The objective of this study is to clarify the effect of light reduction by sediment deposition on leaves and sand movement on eelgrass distribution for obtaining fundamental knowledge on the formation of the eelgrass (Zostera marina L.) habitat. We investigated light regime and sediment deposition on leaves near the lower edge of an eelgrass bed, and monitored sand movement and transplanted eelgrass in an unvegetated site near the higher edge. The average light intensity through sediment deposition on eelgrass leaves was estimated to be approximately 3 E·m-2·d-1 at a low depth limit. As sediment deposition on leaves was 0.7 mg·cm-2 <, the light intensity required by eelgrass might be 4 E·m-2·d-1 <. Sand movement at the unvegetated site ranged from -10 cm to 12 cm and was more unstable than that at a natural site. Scouring for two months reached up to 10 cm, and seedlings disappeared at the unvegetated site near the higher edge. However, the number of vegetative shoots in the unvegetated site steadily increased. These results suggest that vegetative shoots can survive within 10 cm washouts and the growth of eelgrass near the higher edge of an eelgrass bed is restricted to vegetative reproduction.

Assessing the potential benthic–pelagic coupling in episodic blue mussel ( Mytilus edulis) settlement events within eelgrass ( Zostera marina) communities

Coastal marine seagrass ecosystems are important nursery grounds for commercially and recreationally important species, and they serve as key settlement and recruitment sites for other species. We investigated several years (2001–2003) where episodic settlement events of blue mussels ( Mytilus edulis) occurred in Barnegat Bay, NJ, USA. Population assessment indicated that blue mussels settled in eelgrass beds ( Zostera marina) in late spring with peak densities exceeding 170,000 m −2. Based on calculated filtration rates of M. edulis, we determined that for at least 53 days in 2001, the density and size distribution of M. edulis were sufficient to filter the water column volume in excess of twice a day, with maximum calculated filtration rates exceeding 8 m 3 water m −2 day −1. While the settlement event in 2001 was very localized, in 2003, the settlement event was considerably more widespread throughout the bay, with maximum settling densities exceeding 175,000 individuals m −2. Associated with these high densities, maximum calculated filtration rates exceeded 15 m 3 water m −2 day −1. This filtration potential may have impeded the localized development of a brown-tide ( Aureococcus anophagefferens) bloom in 2001, which occurred in other regions of the bay, but the widespread settlement event seen in 2003 may have impeded the development of any brown-tide blooms in Barnegat Bay during that summer. The decline in mussel densities throughout the summer may be a result of elevated water temperatures in this back bay, but at one site, the high settlement of M. edulis was followed by a substantial migration (>40 individuals m −2) of small sea stars ( Asterias forbesii). In 2001, A. forbesii was a significant factor in reducing M. edulis density by the end of the summer at the Barnegat Inlet site and a community level assessment showed significant positive correlations between mussel aggregations and sea star densities ( r=0.68–0.73, P<0.001). At this same site in 2003, the sea stars were again present in high densities (26 m −2) and were a potential mechanism for mussel decline. In other regions of the bay, sea star densities are very low, but numerous other predatory species exist, including blue crabs ( Callinectes sapidus), green crabs ( Carcinus maenus), spider crabs ( Libinia spp.), and several Xanthid crabs. Given the high mussel densities seen in this study and the considerable predation by sea stars and other benthic predators, the benthic–pelagic coupling which these mussels provide in this system contributes to the high secondary production in these grass beds.

Partitioning of food resources among Sillago japonica, Ditremma temmincki, Tridentiger trigonocephalus, Hippocampus japonicus and Petroscirtes breviceps in an eelgrass, Zostera marina, bed

We carried out dietary analysis on five numerically abundant fishes, Sillago japonica, Ditremma temmincki, Tridentiger trigonocephalus, Hippocampus japonicus and Petroscirtes breviceps in an eelgrass bed in Kwangyang Bay, Korea. Comparisons between species demonstrated that the dietary composition of each fish species was significantly different from that of every other species. Although gammarid amphipods and caprellid amphipods were consumed by all species, their individual contributions to each species' diet varied. Furthermore, polychaetes contributed to the diets of S. japonica and T. trigonocephalusand crab larvae were consumed byD. temmincki. Algae and eelgrass were not consumed by four fish species and made only a minimal contribution to the diet of P. breviceps. The diet of each fish species except H. japonicus underwent size-related changes; smaller fishes consumed gammarid amphipods, mysids and copepods, while larger fishes ate polychaetes, gastropods, isopods and other fishes. Differences in the prey organisms consumed of each individual species could be often related to differences in mouth length and width. S. japonica, D. temmincki, T. trigonocephalus, and P. breviceps underwent also a significant diel changes that could be related to differences in foraging behavior and/or prey availability. Thus, use of vision to detect prey would account for the greater daytime consumption of copepods by S. japonica and of crab larvae by D. temmincki, whereas the nocturnal emergence of gammarid amphipods, polychaetes and isopods from the substrate explained their greater consumption by S. japonica, D. temmincki, T. trigonocephalusand P. breviceps at night. Dietary breadth was greater for species with larger mouth dimensions.

Deterioration of eelgrass (Zostera marina L.) through destructive grazing by the gastropod Rissoa membranacea (J. Adams)

Deterioration of an eelgrass (Zostera marina) bed was observed in Skagerrak, southern Norway in August 2002. Compared with observations from previous years, there was a significant reduction in canopy height and shoot density. Simultaneously, high numbers of the gastropod Rissoa membranacea were observed. The proposed mechanism for this drastic loss of eelgrass leaves involves epidermal injuries caused by the grazing activity of the numerous snails, with subsequent leaf breakage. After 1 year, the canopy height had recovered, but shoot density was at the same level as in 2002.

Feeding Ecology of Sillago japonica in an Eelgrass (Zostera marina) Bed

Feeding habits of Sillago japonica collected from in an eelgrass (Zostera marina) bed in Jindong Bay, Korea were studied. S. japonica was a carnivore which consumed mainly gammarid amphipods, polychaetes, bivalves, caridean shrimps and crabs. Its diets also included a small amount of fishes, copepods and caprellid amphipods. The diet of S. japonica underwent significant size-related changes; small individuals (<5cm SL) fed mainly on gammarid amphipods and crab larvae, while proportion of polychaetes and bivalves increased with increasing fish size and gammarid amphipods were also important prey for medium size individuals (5.1-9.9cm SL). The large individuals (>10cm SL) ate polychaetes, caridean shrimps and crabs. The dietary breadth of S. japonica were varied with size. The diet of S. japonica also underwent seasonal changes that could be related to differences in prey availability; gammarid amphipods were mainly consumed in spring and polychaetes in summer.

Relationships between bed age, bed size, and genetic structure in Chesapeake Bay (Virginia, USA) eelgrass (Zostera marina L.)

Genetic structure and diversity can reveal the demographic and selective forces to which populations have been exposed, elucidate genetic connections among populations, and inform conservation strategies. Beds of the clonal marine angiosperm Zostera marinaL. (eelgrass) in Chesapeake Bay (Virginia, USA) display significant morphological and genetic variation; abundance has fluctuated widely in recent decades, and eelgrass conservation is a major concern, raising questions about how genetic diversity is distributed and structured within this metapopulation. This study examined the influence of bed age ( 100ha versus<10ha) on morphological and genetic (allozyme) structure and diversity within Chesapeake Bay eelgrass beds. Although both morphology and genetic diversity varied significantly among individual beds (FST=0.198), neither varied consistently with bed age or size. The Chesapeake eelgrass beds studied were significantly inbred (mean FIS=0.680 over all beds), with inbreeding in old, small beds significantly lower than in other bed types. Genetic and geographic distances within and among beds were uncorrelated, providing no clear evidence of isolation by distance at the scale of 10's of km. These results suggest that local environmental conditions have a greater influence on plant morphology than do bed age or size. They support the hypotheses that eelgrass beds are established by multiple founder genotypes but experience little gene flow thereafter, and that beds are maintained with little loss of genetic diversity for up to 65 years. Since phenotypic and genotypic variation is partitioned among beds of multiple ages and sizes, eelgrass conservation efforts should maximize preservation of diversity by minimizing losses of all beds.

離れ島状に形成されたアマモ場の消長と波浪環境

In open coastal sea area in which turbulence by ocean waves constitutes severe conditions for growth of eelgrass, eelgrass grows densely and maintains its own vegetation by stabilizing the sandy substrate. The larger the scale of vegetation, the more effective is this function, but on the other hand, in forming eelgrass beds, it is more effective to allow eelgrass to form in small patchy beds interspersed with bare areas as much as possible. In order to obtain the basic data for formulating this kind of efficient eelgrass bed formation plan, the authors are conducting field observation surveys. The target site is an extremely small-scale eelgrass bedformed like a remote island in the sea, and the prosperity and decay of the eelgrass bed has been observed and surveyed several times a year for the last three years and nearly every month in 2003. In addition, a wave environment that exerts a great influence over the prosperity and decay of eelgrass was observed at that spot.

Variability in abundance and morphological attributes of Zostera marina L. from the southern coast of Korea

The biomass and morphometric features of subtidal eelgrass (Zostera marina) beds were examined in Kwangyang Bay on the south coast of Korea. Benthic samples were collected from four sites at 2-month intervals, and various environmental parameters were simultaneously monitored. Shoot density increased from 15.7±1.4 (mean±SE) in November 1998 to 65.0±6.0 m-2 in May 1999 and gradually decreased during July–September 1999. Total biomass (shoot and rhizome-root) ranged from 20.2±5.5 in September 1999 to 188.5±19.1 g dry weight m-2 in May 1999. Shoot biomass and density were positively correlated, showing a spring peak on an annual cycle. Significant differences in leaf number per shoot, shoot length, third youngest leaf length, leaf width, leaf surface area index, internode length, root length and root number per node were observed among the sampling times. Shoot length and third youngest leaf length were positively correlated with leaf width, but poorly correlated with the leaf number per shoot. Seasonality of eelgrass beds and morphological traits of Zostera marina at Kwangyang Bay were compared to those in other temperate regions of the northern hemisphere.

Experimental studies of predation by bluefish Pomatomus saltatrix in varying densities of seagrass and macroalgae

We conducted ecosystem level experiments to investigate predator-prey relationships in 3 habitat types; eelgrass Zostera marina, macroalgae, and bare sediment. Mesocosms containing marine sediments, coastal water, and varying densities of eelgrass (75 to 1000 shoots m -2 ) or macroal- gae (50 to 1000 g wet weight m -2 ) were used to simulate conditions found in shallow coastal lagoons of Rhode Island, USA. We used bluefish Pomatomus saltatrix as predators in each experiment after appropriate acclimation in each habitat type. Prey fish were provided in equally weighted rations and included silversides Menidia menidia, tautog Tautoga onitis, cunner Tautogolabrus adspersus, and menhaden Brevoortia tyrranus. Eelgrass significantly increased the survivorship of silversides, tautog and cunner at very low shoot densities. Experiments using macroalgae did not result in significantly different survival rates between bare sand and macroalgae habitat for silversides or cunner, the 2 species tested for this vegetation type. With the decline of eelgrass and increase of macroalgal cover in shallow coastal areas of Rhode Island, this information increases our understanding of the relative value of eelgrass habitat to fish stocks and should provide added incentive to protect and conserve existing eelgrass beds.

Spatial and temporal variation in eelgrass ( Zostera marina) landscapes: influence of physical setting

The distribution of eelgrass ( Zostera marina) was mapped from aerial photographs at three Danish sites with different levels of physical exposure by digital image analysis at 1 m × 1 m resolution. A set of indices from landscape ecology were used to quantify various components of the eelgrass landscapes such as patch shapes and landscape aggregation in photographs from 1995. Furthermore, the long-term fluctuations in the spatial distribution of eelgrass were investigated during the period 1954–1995/1999. Eelgrass landscapes exposed to wave dynamics were less aggregated with more elongated patch shapes than the sheltered eelgrass areas. The outline of the larger patches also tended to be more complex at exposed sites. Patch size distribution at all sites was highly skewed with 75–95% of the patches being <10 m 2. Although the majority of the patches were small, they only constituted a small proportion of the total eelgrass area because most of the eelgrass-covered area was contained in a few, large patches. Large fluctuations in the spatial distribution of eelgrass occurred at both exposed and protected sites in the course of the investigation period. In the relatively short intervals of ∼7 years between two consecutive photographs, on average 39–62% of the total eelgrass area covered in the photographs had changed (i.e., disappeared or recolonized) and the largest changes occurred at exposed sites. Overall, this study showed that shallow eelgrass populations form characteristic landscapes with a configuration that is highly related to the level of physical exposure and that the size and position of eelgrass beds changes substantially among years.

광양만 잘피밭에 서식하는 문치가자미 (Limanda yokohamae)의 식성

Feeding habits of Limanda yokohamae collected from the eelgrass bed in Kwangyang Bay were studied. L. yokohamae (1-16 cm SL) was a carnivore which consumed mainly polychaetes. Its diets included a significant quantity of amphipods (gammarids and caprellids) as well as small quantities of gastropods and ophiuroids. L. yokohamae showed ontogenetic changes in feeding habits. Small individuals less than 4 cm SL preyed mainly on amphipods. However, polychaetes were heavily selected with increasing fish size while the portion of the diet attributable to amphipods decreased sharply. Polychaetes were the major prey organisms for all seasons. Dietary breadth of each size class shows relatively low value, and this means that L. yokohamae depends on only few kinds of food organisms.

The fishes associated with three intertidal salt marsh creeks in a temperate southern African estuary

This study compares the fish assemblages in a range of intertidal salt marsh creeks in the Kariega Estuary, South Africa, as well as highlighting any differences between the ichthyofaunal structure of the creeks and adjacent Zostera bed habitats. The superimposition of environmental variables on the creek biotic groupings (60% similarity level) indicated consistent relationships between both the creek water depth and mouth area with the major creek fish clusters. Water temperature, salinity and turbidity did not display any pattern which could explain the disparity between the ichthyofaunal groupings. The results also indicated that intertidal creeks form a unique littoral habitat within the Kariega Estuary, with ichthyofaunal compositions very different to those found in nearby eelgrass beds. Furthermore, intertidal creeks in the Kariega system appear to be similar to creeks found elsewhere in the world in that their fish assemblages are dominated by 0+ juveniles, have a conspicuous marine transient and estuarine resident component, and very few piscivorous representatives. These results also give weight to the hypothesis that southern African salt marsh creek habitats serve as temporary refuges to juvenile fishes, a role that has also been proposed in other parts of the world.

광양만 잘피밭에 서식하는 꼼치 (Liparis tanakai) 유어의 식성

Feeding habits of juvenile Liparis tanakai collected from the eelgrass bed in Kwangyang Bay were studied. L. tanakai (2.0-6.0 cm SL) was a carnivore which consumed mainly amphipods (caprellids and gammarids) and caridean shrimps. Its diets also included a large amount of copepods, polychaetes and small fishes. Three distinct ontogenetic feeding groups were noted: (1) the individuals of 2.0-2.5 cm SL preyed heavily on copepods, (2) individuals of 2.5-4.0 cm SL fed mainly on amphipods. (3) individuals over 4.0 cm SL preyed mainly on caridean shrimps and small fishes. The dietary breadth of each size class showed relatively low value, and this means that juvenile L. tanakai depends on only few kinds of food organisms.

Changes in the abundance of the seagrassesZostera marina L. (eelgrass) andRuppia maritima L. (widgeongrass) in San Diego, California, following and El Niño Event

Changes in environmental conditions can be accompanied by shifts in the distribution and abundances of organisms. When physical factors become unsuitable for growth ofZostera marina (eelgrass), which is a dominant seagrass species in North America, other more ruderal seagrass species, includingRuppia maritima (widgeongrass), often increase in abundance or replace the dominant species. We report the proliferation of widgeongrass into eelgrass beds in Mission Bay and San Diego Bay in San Diego, California, during the 1997 to 1998 El Nino Southern Oscillation (ENSO). Widgeongrass persisted in these eelgrass beds at least one year after a return to non-ENSO conditions and an increase in eelgrass density. We suggest that a warming of the water in two bays in San Diego by 1.5–2.5°C could result, in a permanent shift in the local seagrass vegetation from eelgrass to widgeongrass. This shift, could, have substantial ecosystem-level ramifications.

防波堤建設による環境変化と移植アマモ場の拡大

The objective of this study is to clarify the effect of wave control by the construction of breakwater and eelgrass transplantation on eelgrass bed formation. We investigated the changes in environmental factors after the construction of the breakwater and transplanted eelgrass bed at the coast of Shimokamagari Island in the Seto Inland Sea, Japan. The rafting reproductive shoots with seeds in natural beds were predicted to be transported by the current to the transplantation area before the construction of the breakwater. However, the eelgrass bed was not formed. Wave height at the transplantation area was predicted to decrease to less than 1/5 due to the construction of the breakwater. This value was the same as those obtained for natural beds. Sediment inside the breakwater was muddier than outside and had an optimum grain size for eelgrass. Sea bottom level was stable inside the breakwater, whereas scouring that uprooted eelgrass occurred outside the breakwater in winter. The transplanted bed inside the breakwater expanded from 2,000m2 to 4,200m2 in two years. The moderate reduction in wave height and sediment stability caused by the construction of the breakwater played an important role in eelgrass bed formation and eelgrass transplantation.

Meristematic oxygen variability in eelgrass (Zostera marina)

We examined the variability in oxygen content of meristematic tissues in eelgrass in order to evaluate its potential role in events of sudden mass mortality within eelgrass beds. Oxygen content within intact eelgrass plants could be described by use of microelectrode techniques at high temporal and spatial resolution in the laboratory and in the field. Under both situations, the meristematic oxygen content was highly variable, ranging from 0 to 200% of air saturation depending on environmental conditions. Changes from steady‐state maximum oxygen content to steady‐state minimum content occurred within 30 min following experimental manipulation. The internal oxygen content exceeded water column oxygen concentration in the light and was intimately coupled to changes in irradiance because of the photosynthetic oxygen release within the leaves. The photosynthetically produced pool of oxygen could, however, not function as an efficient storage to support nighttime respiration. In the dark, oxygen was primarily supplied from the water column via diffusion into leaves, and the meristem quickly turned anoxic if the water column was anoxic. Experimental reduction of oxygen conditions immediately around the basal plant meristem had no major effect on internal oxygen content. High temperatures had a dramatic effect on the internal oxygen balance of eelgrass. Increasing temperature stimulated plant respiration more than photosynthesis, and the meristem went anoxic, even in the light, at water temperatures above 30°C. We hypothesize that low meristematic oxygen content is a key factor in events of seagrass die‐off.

Mortality and productivity of eelgrass Zostera marina under conditions of experimental burial with two sediment types

Mortality and productivity of Zostera marina L. were assessed to examine the effects of experimental burial using 2 types of sediment: (1) sand (6 and 0.2% silt-clay and organic matter con- tent, respectively); and (2) silt (27 and 3.3% silt-clay and organic matter content, respectively). Z. marina was buried to 0, 25, 50, 75 and 100% of its average aboveground height (16 cm) in an exist- ing eelgrass bed using 2 types of sediment characterized as either silty or sandy. Increasing percent- ages of plant burial significantly increased mortality and decreased productivity. Survival and pro- ductivity of eelgrass were substantially reduced when only 25% of the plant height was buried. Plants buried 75% or more of their height were characterized by survival and productivity measures of 0. No statistically significant differences in plant mortality or productivity were found between the 2 sediment types in this experiment. Changes in morphology of the plants were detected in measures of leaf length and surface area in a short duration (12 d) trial of the experiment, apparently in response to senescence, but etiolation was not observed. Results of this experiment indicate Z. marina can only tolerate rapid sedimentation events that cover less than half of its photosynthetic surfaces. Furthermore, the lowest levels of burial treatments (25% of plant height) resulted in mor- tality greater than 50%, indicating that even this small level of rapid sedimentation is significantly detrimental to Z. marina.

Native eelgrass Zostera marina controls growth and reproduction of an invasive mussel through food limitation

In southern California, native eelgrass Zostera marina L. and the invasive non-native mussel Musculista senhousia have dynamic complex interactions. Although high densities of M. sen- housia inhibit the growth of eelgrass, mussel survival and growth decline with increasing eelgrass shoot density and patch size. The correlation of these eelgrass attributes with local concentrations of chlorophyll a and water flow speeds suggested that the mussels, which feed on phytoplankton deliv- ered by water currents, might suffer food limitation inside large eelgrass beds. By supplementing phytoplankton to M. senhousia living in eelgrass, we confirmed this hypothesis: mussels grew 50% faster when food availability was enhanced. Lab and field experiments investigating the effects of food limitation on growth and reproduction of M. senhousia showed that mussels respond by reduc- ing shell growth, and subsequently fecundity. We found no evidence that mussels reallocated resources preferentially to reproduction when food was limited. Our results highlight how the effects of anthropogenic perturbations that currently threaten eelgrass populations directly could be magni- fied by interactions with a non-native species. Eelgrass habitat fragmentation and increasingly frequent phytoplankton blooms resulting from coastal development and eutrophication have well- described direct negative effects on eelgrass. By increasing phytoplankton availability to M. sen- housia, such perturbations also indirectly affect eelgrass by acting to enhance mussel survival and growth, magnifying the negative effects of the mussel on Z. marina.