Secondary production within a seagrass bed (Zostera marina andRuppia maritima) in lower Chesapeake Bay

Monthly sampling of a 140-ha seagrass bed in the lower Chesapeake Bay, Virginia, revealed that 13 numerically and trophically important species, representing about 20% of the total community densities over the year-long study period, accounted for the production of ≈42 g dry wt m^?2 yr^?1. This estimate is likely conservative due to our assumptions on voltinism and fixed size at maturity regardless of season for the species studied. The isopodErichsonella attenuata accounted for 17.6 g dry wt m^?2 yr^?1 or 42% of the calculated total production, while the portunid decapodCallinectes sapidus and the amphipodGammarus mucronatus each accounted for 7.7 g dry wt m^?2 yr^?1. The 10 remaining species (4 peracarids, 4 molluscs, and 2 decapods) each produced less than 2 g dry wt m^?2 yr^?1. Total seagrass-associated secondary production was estimated to equal or exceed 200 g dry wt m^?2yr^?1. By applying this estimate to the entire 140-ha grassbed, we projected that, on average, 4.8 metric tons dry wt of invertebrate standing stock and 55.9 metric tons of invertebrate production occur over the year.     

Seasonal Growth and Senescence of a Zostera marina Seagrass Meadow Alters Wave-Dominated Flow and Sediment Suspension Within a Coastal Bay

Tidally driven flows, waves, and suspended sediment concentrations were monitored seasonally within a Zostera marina seagrass (eelgrass) meadow located in a shallow (1–2 m depth) coastal bay. Eelgrass meadows were found to reduce velocities approximately 60 % in the summer and 40 % in the winter compared to an adjacent unvegetated site. Additionally, the seagrass meadow served to dampen wave heights for all seasons except during winter when seagrass meadow development was at a minimum. Although wave heights were attenuated across the meadow, orbital motions caused by waves were able to effectively penetrate through the canopy, inducing wave-enhanced bottom shear stress (τ _b ). Within the seagrass meadow, τ _b was greater than the critical stress threshold (=0.04 Pa) necessary to induce sediment suspension 80–85 % of the sampling period in the winter and spring, but only 55 % of the time in the summer. At the unvegetated site, τ _b was above the critical threshold greater than 90 % of the time across all seasons. During low seagrass coverage in the winter, near-bed turbulence levels were enhanced, likely caused by stem–wake interaction with the sparse canopy. Reduction in τ _b within the seagrass meadow during the summer correlated to a 60 % reduction in suspended sediment concentrations but in winter, suspended sediment was enhanced compared to the unvegetated site. With minimal seagrass coverage, τ _b and wave statistics were similar to unvegetated regions; however, during high seagrass coverage, sediment stabilization increased light availability for photosynthesis and created a positive feedback for seagrass growth.     

Abundance of seagrass (Zostera marina L.) and macroalgae in relation to the salinity-temperature gradient in Yaquina Bay, Oregon, USA

The relative abundances of the seagrass,Zostera marina L., and associated macroalgae were examined for Yaquina Bay, Oregon, U.S.A., to investigate variability in autotroph abundance along the salinity-temperature gradient and the potential for nuisance algal blooms. Possible explanations for the patterns in autotroph abundances were explored through examination of their correlations with the physicochemical characteristics of the water column. Study sites were established in each of three zones in the estuary defined by temperature and salinity and were sampled monthly June through September 1998 and in July 1999.Z. marina and macroalgal cover andZ. marina shoot density were measured in 0.25-m² plots at each site. After cover estimates and shoot counts were made, material was harvested for determination ofZ. marina and macroalgal biomass. Water column variables were measured from stations near each study site and composited on a depth-averaged, monthly basis for each zone. BothZ. marina and green macroalgal abundance differed between sites, over the summer in 1998, and between years. Seasonal patterns were most obvious forZ. marina at the site closest to the ocean while the pattern in macroalgal abundance suggested a bloom moving up river as summer progressed. The physicochemical characteristics of the zones differed with the season and could be related to the patterns inZ. marina and macroalgal abundance. In particular, salinity was positively correlated withZ. marina abundance, while abundance of both autotrophs was related to light availability.Z. marina biomass ranged 19–109 g dry weight m^?2; green macroalgae biomass ranged 5–234 g dry weight m^?2. The biomass of the green macroalgae at several sites and dates equaled or exceed that of theZ. marina suggesting the potential for nuisance algal blooms does exist in Yaquina Bay.     

Habitat associations of estuarine species: Comparisons of intertidal mudflat, seagrass (Zostera marina), and oyster (Crassostrea gigas) habitats

The complexity of habitat structure created by aquatic vegetation is an important factor determining the diversity and composition of soft-sediment coastal communities. The introduction of estuarine organisms, such as oysters or other forms of aquaculture, that compete with existing forms of habitat structure, such as seagrass, may affect the availability of important habitat refugia and foraging resources for mobile estuarine fish and decapods. Fish and invertebrate communities were compared between adjacent patches of native seagrass (Zostera marina), nonnative cultured oyster (Crassostrea gigas), and unvegetated mudflat within a northeastern Pacific estuary. The composition of epibenthic meiofauna and small macrofaunal organisms, including known prey of fish and decapods, was significantly related to habitat type. Densities of these epifauna were significantly higher in structured habitat compared to unstructured mudflat. Benthic invertebrate densities were highest in seagrass. Since oyster aquaculture may provide a structural substitute for seagrass being associated with increased density and altered composition of fish and decapod prey resources relative to mudflat, it was hypothesized that this habitat might also alter habitat preferences of foraging fish and decapods. The species composition of fish and decapods was more strongly related to location within the estuary than to habitat, and fish and decapod species composition responded on a larger landscape scale than invertebrate assemblages. Fish and decapod species richness and the size of ecologically and commercially important species, such as Dungeness crab (Cancer magister), English sole (Parophrys vetulus), or lingcod (Ophiodon elongatus), were not significantly related to habitat type.     

Phytoplankton production and seasonal biomass variation of seagrass,Ruppia maritima L., in a tropical Mexican lagoon with an ephemeral inlet

Plankton metabolism andRuppia maritima biomass were measured seasonally during 1982–83 in El Verde Lagoon, a small coastal lagoon with an ephemeral inlet on the Pacific Coast of Mexico. Total net aquatic primary production was 521 g C m^?2 y^?1. The water column was slightly heterotrophic, with an annual P/R ratio of 0.89. Our analysis indicates that tropical and subtropical coastal lagoons with restricted or seasonal inlets have generally higher net aquatic primary productivity levels than lagoons with permanently open inlets. We hypothesize that this is due to retention of nutrients and plankton stocks during the dry season. The seasonal pattern of water column metabolism was related to rainfall and riverflow, with higher values generally occurring during the wet season. Net production and respiration were about three times lower during the 1982 dry season as compared to the 1983 dry season which received considerable rains due to abnormal climatic conditions. The biomass ofR. maritima ranged from zero to 620 g dry wt m^?2. Growth occurred only during the dry season and there were two distinct biomass peaks representing two separate crops. The second crop was heavily epiphytized with nitrogen-fixing algae. There was an apparent succession in dominance of water column productivity over the year, withRuppia dominating during the dry season and phytoplankton more important during the wet season.     

Sources of Salinity Variation in a Coastal Lagoon in a Karst Landscape

Bahia de la Ascension (BA) is a shallow, mangrove-fringed coastal bay connected to the Caribbean through two inlets, outlined by the Mesoamerican Barrier Reef System. This work represents an initial investigation of the relative contribution of hydrometeorological and hydrodynamic forcing on salinity variation in this lagoon. Our objective is to assess the sensitivity of the salinity in BA to fluctuations in freshwater inflow and coastal oceanography. Two field trips were undertaken during rainy and dry seasons in 2007. Surface salinity was mapped across the system and CTD deployments carried out within BA and in the sea end-member to characterize temperature, conductivity, and water level. Also, cross-sectional CTD profiles were implemented to examine vertical stratification. The water balance indicated that 16 % of rainfall over the drainage basin (DB) becomes groundwater discharge plus surface runoff into BA during dry season, while 68 % of the precipitation input to the DB is supplied through groundwater–surface runoff to the bay during rainfalls. This combined inflow showed larger fluctuations than direct rainfall and, thus, has a greater potential to alter the seasonal salinity variations within BA. The tidal signal is selectively attenuated within BA, as diurnal frequencies are more readily filtered out than semidiurnal frequencies. Mesohaline conditions in the southwest bay are associated with freshwater sources, while saline water masses in the inlet are influenced by prevalent SE winds in the region and tidal phase, establishing a strong horizontal SW-NE estuarine salinity gradient.     

The demise and recovery of seagrass in the northern Indian River Lagoon,Florida

Seagrass both disappeared and recovered within 4 yr in one region of northern Indian River Lagoon (IRL). For the specific area referred to as Turnbull Bay, a relatively pristine area of the IRL, over 100 ha of seagrass completely disappeared from 1996 to 1997 and then recovered by 2000. Based on lagoon-wide mapping from aerial photographs taken every 2–3 years since 1986, coverage of seagrass in Turnbull Bay declined from 124 ha in 1989 to 34 ha by 1999 and increased to 58 ha in 2003. Bi-annual monitoring of fixed seagrass transects tells a more detailed story. Species composition along the Turnbull transect shifted fromHalodule wrightii toRuppia maritima beginning in 1995, and macroalgal abundance increased. By the summer of 1997, seagrass completely disappeared along the transect, as well as in most of the surrounding areas in Turnbull Bay; macroalgae covered much of the sediment surface. No significant water quality changes were detected. Light attenuation and suspended solid values did increase after the seagrass disappeared. Porewater sulfide concentrations, taken after all the grass was gone in 1997, were high (2,000 μM), but did improve by 1998 (1,200 μM). Seagrass recovery was rapid and occurred in the reverse sequence of species loss. Seedlings ofR. maritima were the first colonizers, then patches ofH. wrightii appeared. In 2000,Halophila engelmannii returned in the deeper water (>0.6m). By the summer of 2000, the beds had completely recovered. We conclude that this demise was a natural event caused by a long-term buildup of seagrass biomass and a thick (10–15 cm) layer of organic detritus and ooze. We surmise that such a crash and subsequent recovery may be a natural cycle of decline and recovery within this semirestricted, poorly-flushed area. The frequency of this cycle remains uncertain.     

Temporal Variation in the Trophic Levels of Secondary Consumers in a Mediterranean Coastal Lagoon (Cabras Lagoon,Italy)

Trophic levels (TLs) of fish were estimated on three sampling dates (March, May, and August 2006) for different fish sizes in the Cabras Lagoon (Sardinia, Italy). A temporal TL variation for Atherina boyeri, Gobius niger, and Engraulis encrasicolus was observed. In March and May, the TL ranged from 3.3 to 3.4, characterizing these species as secondary consumers, while in August, this range moved to between 3.7 and 3.8, indicating a TL shift towards tertiary consumers. For Liza ramada, TL was consistently lower in small individuals (mean TL 2.5) than in larger individuals (mean TL 3.0). Increased TL of the fish species A. boyeri, G. niger, and E. encrasicolus in August was consistent with the seasonal changes in the macrobenthic assemblage in this system, with a dominance of primary consumers (benthic deposit feeders) in winter–spring and a dominance of secondary consumers (the nereidids Alitta succinea and Hediste diversicolor) in summer. These fish species took advantage of the high availability of nereidids leading to a rise in their TL values. Furthermore, the increase of TL with size of L. ramada, the most economically valuable fish species in the Cabras Lagoon, indicated an ontogenetic diet shift, the juveniles being omnivorous, while the adults being secondary consumers. We conclude that variability in the trophic levels of fish due to species traits, ontogenetic diet shifts, and variation in prey availability should be taken into account to further understand the food web structure of coastal lagoons.     

Exchange of Nitrogen and Phosphorus Between a Shallow Lagoon and Coastal Waters

West Falmouth Harbor, a shallow lagoon on Cape Cod, has experienced a threefold increase in nitrogen load since the mid- to late 1990s due to input from a groundwater plume contaminated by a municipal wastewater treatment plant. We measured the exchange of nitrogen and phosphorus between the harbor and the coastal waters of Buzzards Bay over several years when the harbor was experiencing this elevated nitrogen load. During summer months, the harbor not only retained the entire watershed nitrogen load but also had a net import of nitrogen from Buzzards Bay. During the spring and fall, the harbor had a net export of nitrogen to Buzzards Bay. We did not measure the export in winter, but assuming the winter net export was less than 112 % of the load, the harbor exported less than half of the watershed nitrogen load on an annual basis. For phosphorus, the harbor had a net import from coastal waters in the spring and summer months and a net export in the fall. Despite the large increase in nitrogen load to the harbor, the summertime import of phosphorus from Buzzards Bay was sufficient to maintain nitrogen limitation of primary productivity during the summer. Our findings illustrate that shallow systems dominated by benthic producers have the potential to retain large terrestrial nitrogen loads when there is sufficient supply of phosphorus from exchange with coastal waters.     

Upwind return flow in a Coastal Lagoon: Seasonal-scale barotropic transport

Current and wind data collected during a 223-d study period from early June 1983 through early January 1984 are used to document net upwind return flow over seasonal time scales in the Indian River Lagoon near Sebastian Inlet on the Atlantic Coast of central Florida. Spectral analysis of wind and current meter data suggests that wind forcing accounts for the majority of nontidal flow. The two time series are highly coherent at periodicities of 2,3–3.5, 4, and 6 d. The serial correlation coefficient of ?0.49 is significant at the 99.9% confidence level. Historical water level and wind data recorded simultaneously at Sebastian Inlet and Eau Gallie, 34.5 km to the north, suggest that longitudinal pressure gradients are established in response to seasonal wind patterns. The upwind near-bottom return flow is interpreted as a response to the seasonal set up of the longitudinal pressure gradients. The study documents the importance of windstress in forcing low-frequency transport over seasonal time scales.     

The Decline and Restoration of a Coastal Lagoon (Lake Veere) in the Dutch Delta

The former tidal inlet Lake Veere was turned into a stagnant brackish lake in 1961. Ever since, the system has shown a continuous degradation. The current study shows the monitoring results for the macrozoobenthic communities and the abiotic conditions for the period 1990–2008. This includes the first step to remediation, the restoration of the exchange between the tidal marine Eastern Scheldt and Lake Veere in 2004. A continuous decline in water clearance co-occurring with decreasing macrofauna densities and richness was observed till 2004. Water quality (e.g., secchi depth, nutrient levels, and oxygen conditions) improved significantly after the measure at a higher salinity level with less variation. But the macrofauna densities, biomass, and diversity did not improve yet. First indications of changes in the benthic communities by arrival of new and returning species are however observed and show that restoration at macrofauna level follows the improved abiotic conditions with a delay of several years.     

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Fishes

The structure of the fish community associated with eelgrass beds in the lower Chesapeake Bay was studied over a 14 month period. A total of 24,182 individuals in 48 species was collected by otter trawl with Leiostomus xanthurus (spot) comprising 63% of the collection, Syngnathus fuscus (northern pipefish) 14%, Anchoa mitchilli (bay anchovy) 9%, and Bairdiella chrysoura (silver perch) 5%. The density and diversity of fishes were higher in vegetated areas compared to unvegetated areas; fishes were more abundant in night collections Fish abundance and species number increased in the spring and early summer as both water temperature and eelgrass biomass increased and decreased in the fall and winter as temperature and eelgrass biomass decreased. Gill netting revealed some of the top predators in the system, especially the sandbar shark, Carcharhinus milberti. The fish community in the Chesapeake Bay was quite different from North Carolina eelgrass fish communities. Most notable was the rarity of the pinfish, Lagodon rhomboides, which may be a very important predator in the structuring of the epifaunal communities.     

Major Ion Chemistry in a Freshwater Coastal Lagoon from Southern Brazil (Mangueira Lagoon): Influence of Groundwater Inputs

This paper characterizes major ion distributions and investigates whether groundwater exerts a major control on the chemical functioning of Mangueira Lagoon, a large (90 km long), shallow (∼4–5 m deep), and fresh coastal lagoon in southern Brazil. Water volumes equivalent to ∼80% of the total annual input are used in the summer for irrigating nearby rice plantations, the most important regional economic activity. While Na⁺ and Cl⁻ are the major ions in local groundwater, Na⁺ and HCO₃⁻ are the most enriched ions in lagoon water. The ion concentrations measured in Mangueira Lagoon were homogeneous, except for a few samples affected by rainwater and groundwater inputs. A shore-normal transect starting at the pump house of a rice irrigation canal indicated strong groundwater input at this canal. In spite of the small volume contribution (∼2% of precipitation), groundwater discharge accounts for 50–70% of major ion inputs into the lagoon, with ∼70% of the groundwater inputs being anthropogenically derived (e.g., from the rice irrigation canals). This may have serious implications for the management of the coastal water resources from Mangueira Lagoon and other similar areas as groundwater associated with agricultural systems may be contaminated by fertilizers and pesticides. The results imply that groundwater should not be neglected in dissolved species’ budgets even when its volume contribution is small.     

Regulation of eelgrass (Zostera marina) cover along depth gradients in Danish coastal waters

A large data set, collected under the national Danish monitoring program, was used to evaluate the importance of photon flux density (PFD), relative wave exposure (REI), littoral slope, and salinity in regulating eelgrass cover at different depth intervals in Danish coastal waters. Average eelgrass cover exhibited a bell-shaped pattern with depth, reflecting that different factors regulate eelgrass cover at shallow- and deep-water sites. The multiple logistic regression analysis was used to identify regulating factors and determine their role in relation to eelgrass cover at different depth intervals. PFD, REI, and salinity were main factors affecting eelgrass cover while littoral slope had no significant effect. Eelgrass cover increased with increasing PFD at water depths of more than 2 m, while cover was in versely related to REI in shallow water. This pattern favored eelgrass cover at intermediate depths where levels of PFD and REI were moderate. Salinity had a minor, but significant, effect on eelgrass cover that is most likely related to the varying costs of osmoregulation with changing salinity. The analysis provided a useful conceptual framework for understanding the factors that regulate eelgrass abundance with depth. Although the regression model was statistically significant and included the factors generally considered most important in regulating eelgrass cover, its explanatory power was low, especially in shallow water. The largest discrepancies between predicted and observed values of cover appeared in cases where no eelgrass occurred despite sufficient light and moderate levels of exposure (almost 50% of all observations). These discrepancies suggest that population losses due to stochastic phenomena, such as extreme wind events, played an important regulating role that is not adequately described by average exposure levels. A more thorough knowledge of the importance of such loss processes and the time scales involved in recovery of seagrass populations after a severe disturbance are necessary if we are to understand the regulation of seagrass distribution in shallow coastal areas more fully.     

Structural components of eelgrass (Zostera marina) meadows in the lower Chesapeake Bay—Decapod crustacea

Otter trawl collections of eelgrass habitats in the lower Chesapeake Bay during 1976–1977 produced 14 species of decapod crustaceans. These collections were dominated by palaemonid shrimp (Palaemonetes spp.), blue crabs (Callinectes sapidus), and sand shrimp (Crangon septemspinosa), each of which exhibited unimodal seasonal abundance curves with large summer peaks. Decapod abundance was positively correlated with plant biomass throughout the year. Decapod densities on vegetated bottoms were greater than on unvegetated bottoms, and nighttime abundance on each bottom type was greater than corresponding daytime abundance. Total decapod abundances in Chesapeake Bay eelgrass meadows appear to be much greater than those reported in North Carolina eelgrass or Gulf of Mexico turtlegrass habitats.     

Factors Controlling Phytoplankton Biomass in a Subtropical Coastal Lagoon: Relative Scales of Influence

Patterns in phytoplankton biomass are essential to understanding estuarine ecosystem structure and function and are the net result of various gain and loss processes. In this study, patterns in phytoplankton biomass were explored in relation to a suite of potentially regulating factors in a well-flushed, subtropical lagoon, the Matanzas River Estuary (MRE) in northeast Florida. We examined temporal variability in water temperature, light availability, nutrient concentrations, phytoplankton productivity, and phytoplankton standing stock over 8 years (2003–2010) and explored relationships among variables through correlation analysis. Laboratory experiments in the spring and summer of 2009 quantified phytoplankton growth rates, nutrient limitation potential, and zooplankton grazing rates. The potential influence of oyster grazing was also examined by scaling up population metrics and filtration rate estimates. Results indicated that phytoplankton biomass in the study area was relatively low mainly due to a combination of low temperature and light availability in the winter and consistent tidal water exchange and bivalve grazing throughout the year. Relatively low levels of phytoplankton standing stock and small inter-annual variability within the MRE reflect a balance between gain and loss processes which provide a degree of resilience of the system to natural and anthropogenic influences.     

The autecology and production dynamics of eelgrass (Zostera marina L.) in Netarts Bay,Oregon

Changes in biomass, growth form and shoot net primary production in an eelgrass, Zostera marina L., bed were monitored along transects at three tidal heights in Netarts Bay, Oregon, from May 1979 through June 1981. During the growing season, April through October, the mean plastochrone interval was 16.5 d along the low intertidal transect and 11.6 d along the high intertidal transect. The mean export interval was 13.3 d along the low intertidal transect and 11.6 d along the high intertidal transect. The life span of a leaf averaged 48 d along the low intertidal transect and 36 d along the high intertidal transect. Shoot density was positively correlated with mean leaf area index (LAI) until the LAI reached 3.8 to 5.5, above which LAI was negatively correlated with density. The maximum Zostera biomass ranged from 143 (high intertidal transect) to 463 (low intertidal transect) g dry wt m^?2. Maximum values of shoot net production ranged from 4.7 (high intertidal transect) to 13.6 (low intertidal transect) g dry wt m^?2d^?1. Zostera shoot net production was related to light and to the physical damage to the shoots associated with a rapid accumulation of Enteromorpha biomass in the bay. In addition, patterns of biomass accumulation were related to the duration of water coverage, as determined by both tidal height and local impoundments of water. At all transects, biomass sloughed was equal to at least 50% of the shoot net primary production in that area during that time period; sloughed leaves accounted for 25 to 97% of these losses. An estimate of the total annual net primary production of aboveground Zostera in the bed was 17,500 kg, dry wt (SE=3,080 kg dry wt), which was equivalent to a mean annual rate of 383 g C m^?2 (SE=67 g C m^?2)     

Factors Affecting Macrobenthic Fauna in a Tropical Hypersaline Coastal Lagoon in Ghana,West Africa

The macrobenthic fauna in the large, hypersaline, shallow Keta lagoon in Ghana was sampled at 20 stations in the wet (September 2002) and dry seasons (March 2003) to elucidate the effects of abiotic factors on community patterns. The macrobenthic fauna was low in density and species diversity and numerically dominated by bivalves and capitellid polychaetes. These organisms appear able to withstand physical disturbance (when lagoon water levels become extremely low) and osmotic stress (when salinities are extremely high) and tend to redistribute along environmental gradients. Parallel seasonal differences in several environmental variables and the macrobenthic fauna indicate a highly dynamic system. Species richness and diversity were higher in the wet season than the dry season. Salinity, percent clay, pH, and turbidity in that order were the major significant variables structuring the macrobenthic faunal assemblage in Keta lagoon. The strong effect of seasonal salinity changes on macrobenthic faunal assemblages may have trophic consequences for higher organisms of commercial importance, such as fishes and shorebirds, in the Keta lagoon.     

Seed bank patterns in Chesapeake Bay eelgrass (Zostera marina L.): A bay-wide perspective

The decline of eelgrass (Zostera marina) in Chesapeake Bay in the 1960s and 1970s has been studied in the context of changes in water quality and habitat suitability; little effort has focused on the importance of reproductive ecology in understanding current and potential recovery of these populations. The spatial variability of seed-bank characteristics ofZ. marina in Chesapeake Bay was explored by a reproductive shoot and seed-bank sampling effort. Seed banks were sampled from 105 beds of submerged aquatic vegetation among 12 zones throughout the lower and middle Chesapeake Bay. Number of viable seeds was highly variable among and within zones, with seeds found in all but one zone and also found in cores not containing anyZ. marina shoots. Number of reproductive shoots was also highly variable among and within zones, with differences probably driven by different local environmental conditions. Bay-wide, viable seeds were found in more monospecificZ. marina cores than in mixed species or monospecificRuppia maritima cores suggesting local biological and environmental control on sexual reproduction. Lower densities of viable seeds in the middle Chesapeake Bay region reflect the lower abundance ofZ. marina in these regions and provide context for discussion of historical changes inZ. marina in Chesapeake Bay. While this study focused on a snap shot of the seed bank immediately after establishment, we highlight critical questions for future study that may be important for their conservation and restoration.