Clam Biomass Research Articles

Ecosystem Responses in the Distribution of Black Clam (<i>Villorita cyprinoides</i>) Beds in Vembanad Estuary during Environmental Changes Using GIS and RS

The biomass and distribution of black clam (Villorita cyprinoides) in Vembanad, a tropical estuary located along the southwest coast of India varied significantly. Sampling was done in freshwater-dominated zone in the south (distal) and brackish water zone in the north (proximal), during pre and post monsoon seasons. Clam biomass was estimated from samples, collected from different stations during the study period. Water transparency and temperature were measured at the sample sites. Water samples were collected and analysed for salinity, dissolved oxygen (DO), pH and hardness. There was a significant difference in the clam biomass during the two seasons in the distal zone, and those collected from the distal and proximal zones during pre-monsoon season. The data were further analysed to determine the factors affecting the clam biomass distribution in the two zones and seasons. Factor analyses, comparing the distal zone during two seasons and zonal variations were similar to earlier observations. Step wise regression analyses found that dissolved oxygen (adjusted R2 = 0.3) is the only variable affecting clam survival during pre-monsoon period in the distal and proximal zones. A geographic map of the region obtained from the Indian satellite sensor LISS (Linear Image Self Scanner) was used along with in situ data to map the results using inverse distance weightage model.

The influence of commercial harvesting on parasite infection in the bivalve Austrovenus stutchburyi

Commercial harvesting often reduces densities of target species, potentially affecting transmission of parasites within the population. Rates of parasitic infection not only impact the hosts directly, but can result in wider ecosystem effects through influences on host behaviour. Commercial harvesting may therefore have important ecological consequences beyond the direct effects of removal of biomass. Clams (Austrovenus stutchburyi) are commonly infected by trematode parasites (Acanthoparyphium spp. and Curtuteria australis) and have been commercially harvested in Otago, New Zealand, since 1983. To assess the effects of harvesting on the abundance of trematodes in A. stutchburyi, the number of parasites per clam was compared in harvested and unharvested areas. The mean number of parasites per clam was 36% higher at commercially harvested sites, suggesting that harvesting enhances the local transmission of parasites. Harvesters may be able to reduce their influence on parasite infection levels in clam populations by harvesting less intensively and allowing more time between harvests for clam biomass to regenerate.

Coupling planktonic and benthic shifts during a bloom of Alexandrium catenella in southern Chile: Implications for bloom dynamics and recurrence

Cell abundances and distributions of Alexandrium catenella resting cysts in recent sediments were studied along time at two locations in the Chilean Inland Sea exposed to different oceanographic conditions: Low Bay, which is much more open to the ocean than the more interior and protected Ovalada Island. The bloom began in interior areas but maximum cyst concentrations were recorded in locations more open to the ocean, at the end of the Moraleda channel. Our results showed a time lapse of around 3 months from the bloom peak (planktonic population) until the number of resting cysts in the sediments reached a maximum. Three months later, less than 10% of the A. catenella cysts remained in the sediments. Maximum cyst numbers in the water column occurred one month after the planktonic peak, when no cells were present. The dinoflagellate assemblage at both study sites was dominated by heterotrophic cysts, except during the A. catenella bloom. CCA analyses of species composition and environmental factors indicated that the frequency of A. catenella blooms was associated with low temperatures, but not with salinity, chlorophyll a concentration, and predator presence (measured as clam biomass). However, resting cyst distribution was only related to cell abundance and location. The occurrence of A. catenella cysts was also associated with that of cysts from the toxic species Protoceratium reticulatum. By shedding light on the ecological requirements of A. catenella blooms, our observations support the relevance of encystment as a mechanism of bloom termination and show a very fast depletion of cysts from the sediments (<3 months), which suggest a small role for resting cyst deposits in the recurrence of A. catenella blooms in this area.

Decline of a Callista chione (Bivalvia: Veneridae) bed in the Maresme coast (northwestern Mediterranean Sea)

Bivalves are considered key species in many marine communities and the decline of their populations usually causes changes in the structure of the ecosystems.The aim of the present study was to investigate changes in the spatial distribution and population structure of an exploited shellfish bed of Callista chione at a location of the northwestern Mediterranean Sea and analyse its potential causes. To this end field surveys for the years 2004 and 2010 covering the whole distribution area of the species were performed and all historical data on commercial landings, clam biomass and density, and locations and volume of dredged sand for beach nourishment were compiled. Results showed that the distribution had been hugely reduced, showing a sharp decline in density and biomass and a decrease in the mean population size in 2010. Our results confirm that the species is a slow-growing bivalve that is highly vulnerable to disturbances prolonged in time. Persistent sand dredging operations since 1987 have gradually reduced the distribution area of the smooth clam to the northeastern part of the clam bed. Consequently, fishing activity has concentrated on a smaller area, thus increasing the fishing effort in the northern part. Therefore, the results clearly suggest that the decline of C. chione bed was mainly caused by the interaction of two coastal uses: sand dredging and clam fisheries. Based on this study, some management guidelines are proposed.

Impacts of Corbicula fluminea on Oxygen Uptake and Nutrient Fluxes across the Sediment–Water Interface

Corbicula fluminea (Asian clam) can assemble in high densities and dominate benthic communities. To evaluate the influence of this clam on sediment oxygen uptake and nutrient fluxes across the sediment–water interface, a microcosm study was conducted using a continuous-flow cultivation system with sediment, lake water, and C. fluminea specimens from Taihu Lake, China. C. fluminea destroyed the initial sediment surface, enhanced O2 penetration into the sediment partially, and increased the sediment water content, the volume of oxic sediment, and total microbial activity. Sediment O2 uptake was significantly stimulated by C. fluminea. Linear regression results for O2 uptake versus clam biomass ranged from 21.9 to 9.47 μmol h−1 g−1 DW (dry weight). The release of soluble reactive phosphorus, ammonium, and nitrate from the sediment was also increased by the clams. The increase in the amount of soluble reactive phosphorus and ammonium released into the overlying water was 0.042 ~ 0.091 and 2.77 ~ 3.03 μmol h−1 g−1 DW, respectively, and this increase was attributed to increased diffusion, enhanced advection between the pore water and the overlying water, and the excretions from C. fluminea. Enhanced nitrification was suggested as the reason for the increase in nitrate release (2.95 ~ 4.13 μmol h−1 g−1 DW) to the overlying water.

서해 근소만 조간대 퇴적물에서 여름과 겨울에 인의 존재형태

A sequential extraction technique was used to study sediment phosphorus speciation and its relative importance in the intertidal flat of Keunso Bay during summer and winter for a better understanding of the phosphorus cycle and bioavailability in intertidal sediments. Loosely sorbed P contents were the lowest among the five P-pools and showed little seasonal or spatial variation. Although Fe-bound P contents were almost constant in winter, they decreased rapidly with sediment depth in summer. The dissolution of Fe oxides, used as an oxidant for the anaerobic respiration, ascribed the rapid decrease of Fe-bound P in summer. Al-bound P contents displayed little seasonal variation, but showed a large spatial variation, with higher values in the upper intertidal flat. Comprising about 50% of total P, Ca-bound P contents were the highest among the five P-pools. Ca-bound P contents were higher in winter than summer, but did not exhibit a clear spatial variation. Organic P contents were higher in summer than winter, which was associated with higher primary production and clam biomass in summer. Organic P contents were higher in the lower intertidal flat than the upper intertidal flat. In Keunso Bay, bioavailable P contents of the intertidal flat comprising about one third of total P ranged from 2.41 to 5.09 <TEX>${\mu}molg^{-1}$</TEX> in summer and 3.82 to 5.29 <TEX>${\mu}molg^{-1}$</TEX> in winter. The bioavailability of P contents was higher in the lower intertidal flat than the upper intertidal flat, which was attributed to the large clam production in the lower intertidal flat.

Acute Disturbance of Lake Pontchartrain Benthic Communities by Hurricane Katrina

In 2005, Hurricane Katrina produced a distinct, acute ecological disturbance of the benthic invertebrate community of Lake Pontchartrain, LA, USA. The bivalve Rangia cuneata and other community dominants were lost from 50% (815 km2) of the lake bottom. The storm surge directly killed benthic organisms and produced salinity stratification that caused episodes of detrimental low dissolved oxygen concentration at depths >3.7 m. Past disturbance of the bottom by shell dredging and intrusion of higher salinity bottom water through deep shipping channels appears to have contributed to the severity of this impact. Colonization by tolerant opportunistic taxa occurred, but low rainfall after Katrina has slowed the recovery of the typical rangia community. A decrease in water transparency and an increase in turbidity and chlorophyll a were associated with the loss of clam biomass. Other hurricanes may have produced less obvious but similar effects on smaller spatial and temporal scales.

A modeling study on methylmercury bioaccumulation and its controlling factors

The objectives of this study were: (1) to develop a methylmercury (MeHg) bioaccumulation model using data from STORM (high bottom Shear realistic water column Turbulence Resuspension Mesocosms) experiments; and (2) to use the model as a diagnostic tool to examine an effect of sediment resuspension and other important factors on MeHg bioaccumulation. There were four mesocosm experiments (1–4) conducted both in summer and fall. Tidal resuspension (4 h on- and 2 h off-cycles) was simulated using the STORM facility at CBL, UMCES (Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science). The model results showed that changes in clam biomass had a great effect on phytoplankton and zooplankton biomass, and consequently MeHg accumulation. In addition, it appeared that sediment resuspension played a role in transferring the enhanced sediment MeHg into organisms inhabiting both water column and sediment.

A spatially resolved model of seasonal variations in phytoplankton and clam ( Tapes philippinarum) biomass in Barbamarco Lagoon, Italy

Barbamarco Lagoon (area = 7 km 2) is in the Po River Delta, adjoining the Northern Adriatic Sea, and supports a commercially valuable clam ( Tapes philippinarum) fishery. This study investigated interactions of the lagoon with adjacent coastal waters and inland riverine inputs by modelling both the lagoon and the Northern Adriatic Sea, using a coupled three-dimensional (3D) hydrodynamic-ecological model (ELCOM-CAEDYM) adapted to include the clam population. The clam model accounted for carbon (C), nitrogen (N) and phosphorus (P) biomass in the benthos through parameterisations for filtration, excretion, egestion, respiration, mortality, and harvesting. Multiple clam size classes were included in a new population dynamics sub-model. Output from the coupled model was validated against hydrodynamic and water quality data from intensive field sampling and routine monitoring. Time scales of tidal flushing, primary production and clam grazing were investigated with the model to demonstrate that food supply to clam populations is dominated by phytoplankton inputs from the Northern Adriatic Sea. Effects of clam cultivation on nutrient concentrations and phytoplankton biomass in Barbamarco Lagoon were primarily localised, with strong tidal flushing minimising impacts of clam filtration on lagoon-wide nutrient concentrations at current clam stocking levels. Clam populations were found to alter the cycling of nutrients in the system, causing the lagoon to become a net sink for particulate organic matter and to export dissolved organic matter to the adjacent sea via tidal flushing. Ecosystem health and sensitivity of nutrient cycles to clam cultivation are important considerations for the long term sustainable management and potential expansion of the fishery.

FORAGING BEHAVIOR OF SURF SCOTERS (MELANITTA PERSPICILLATA)AND WHITE-WINGED SCOTERS (M. FUSCA) IN RELATION TO CLAM DENSITY: INFERRING FOOD AVAILABILITY AND HABITAT QUALITY

Abstract Birds are known to modify their foraging behavior in relation to food availability. Once understood, these relationships can be used to draw inferences about relative food availability and habitat quality. We measured foraging behavior of Surf Scoters (Melanitta perspicillata) and White-winged Scoters (M. fusca) feeding on clams during winter in the Strait of Georgia, British Columbia, to evaluate the relative quality of the foraging landscape for wintering scoters. Because clam biomass does not increase appreciably during winter through growth or recruitment, scoters are faced with a depleting and potentially exhaustible food supply. Along with this temporal variation, clam densities vary widely by site. We considered the influence of variation in clam density on scoter foraging behavior, along with other factors known to affect foraging in other sea duck species, such as season, sex, age, and environmental attributes. Clam-capture success (clams captured per dive) and foraging effort (minutes u...

POPULATION DENSITY, BIOMASS, AND AGE-CLASS STRUCTURE OF THE INVASIVE CLAM CORBICULA FLUMINEA IN RIVERS OF THE LOWER SAN JOAQUIN RIVER WATERSHED, CALIFORNIA

Abstract Corbicula fluminea is well known as an invasive filter-feeding freshwater bivalve with a variety of effects on ecosystem processes. However, C. fluminea has been relatively unstudied in the rivers of the western United States. In June 2003, we sampled C. fluminea at 16 sites in the San Joaquin River watershed of California, which was invaded by C. fluminea in the 1940s. Corbicula fluminea was common in 2 tributaries to the San Joaquin River, reaching densities of 200 clams · m−2, but was rare in the San Joaquin River. Biomass followed a similar pattern. Clams of the same age were shorter in the San Joaquin River than in the tributaries. Distribution of clams was different in the 2 tributaries, but the causes of the difference are unknown. The low density and biomass of clams in the San Joaquin River was likely due to stressful habitat or to water quality, because food was abundant. The success of C. fluminea invasions and subsequent effects on trophic processes likely depends on multiple factors....

Modeling the foodweb in coastal areas: a case study of Ringkøbing Fjord, Denmark

AbstractThis work utilizes the CoastWeb model, a foodweb model for coastal areas that also includes a mass‐balance model (CoastMab) for phosphorus and many abiotic/biotic interactions, to study the development in Ringkøbing Fjord, Denmark, from 1985 to 2004. This shallow coastal lagoon has an area of 300 km2 and a mean depth of 1.9 m. The water exchange between the lagoon and the North Sea is regulated by a sluice. In 1996 there was a major regime shift in this lagoon with drastic reductions in chlorophyll‐a concentrations, significant increases in water clarity (Secchi depth) and major changes in the number and biomass of clams as well as in macrophyte cover. Regime shifts is a “hot” topic in aquatic ecology and in this work the CoastWeb model is used as a tool to understand and quantify the causes behind this regime shift. The CoastWeb model is general and can also be used for other coastal areas. The basic model calculates monthly production values and changes in biomasses of ten functional groups of organisms (phytoplankton, bacterioplankton, herbivorous, and predatory zooplankton, benthic algae, macrophytes, jellyfish, zoobenthos and prey and predatory fish) and in Ringkøbing Fjord, also for clams (Mya arenaria). In spite of its complexity, the model is relatively simple to use, since all driving variables may be readily accessed from maps or monitoring programs. The model includes much abiotic/biotic feedback and it can also be used to address other causes for regime shifts other than the changes in salinity and nutrient inflow, which have caused the changes in Ringkøbing Fjord. The model has previously been tested for more than 20 smaller coastal areas and was shown to predict variations in foodweb characteristics very well. The focus of this paper is on temporal variations within one well‐studied coastal area. The paper compares modeled values to empirical data for Ringkøbing Fjord and discusses fundamental ecosystem features such as regime shifts and compensatory effects in a way that is not practically feasible without the use of quantitative models.

Interplay between filter-feeding zoobenthos and hydrodynamics in the shallow Odense Fjord (Denmark) – Earlier and recent studies, perspectives and modelling

The shallow Odense Fjord (Denmark) is characterized by a large biomass of filter-feeding polychaetes ( Nereis diversicolor), clams ( Mya arenaria), cockles ( Cerastoderma glaucum), and amphipods ( Corophium volutator). The present paper summarizes studies on zoobenthic filter-feeding in Odense Fjord from the last 10 years. The general principles discovered are extracted and compared to available tools for modelling of the primary characteristics of interplay between benthic filter-feeders and hydrodynamics. Earlier works have been supplemented with data from a recent field study conducted in the shallow inner part of the fjord. Based on data from this study site, the reduction in phytoplankton for fully mixed and incompletely mixed flows has been modelled. It was found that fully mixed flow results in a potential half-life for phytoplankton of only 1.3 h, whereas for the incompletely mixed water the half-life is 2.7 times longer. The field measurements clearly demonstrate the presence of a strong interplay between filter-feeders and hydrodynamics, but although a certain grazing impact is evident from vertical chlorophyll a profiles with often strongly reduced near-bottom concentrations it is not straightforward to identify and model even the main bio-physical processes that prevent the dense populations of filter-feeders to completely control the phytoplankton biomass in Odense Fjord.

Modelling phosphorus and suspended particulate matter in Ringkøbing Fjord in order to understand regime shifts

The focus in this work is on the conditions in Ringkøbing Fjord (Denmark), where there have been major regime shifts during the last 30 years. The regime shift in the 1990s is discussed in this paper and concerns reductions in nutrient input from land and changes in salinity. The changes in these abiotic have drastically influenced the structure and functioning of the ecosystem [e.g., in phytoplankton production, water clarity, macrophyte cover and biomass of clams]. This work concerns the modelling and understanding of such changes and the aim is also to consider patterns in variability in the data that may explain the regime shift. The model used is a general, dynamic process-based mass-balance model for total phosphorus (TP) and suspended particulate matter (SPM). The model uses ordinary differential equations (the ecosystem scale). The calculation time is 1 month to reflect seasonal variations. We quantify, e.g., sedimentation, resuspension, diffusion, mixing and retention. The model has previously been tested for coastal areas of different character and shown to predict TP, SPM, Secchi depth, chlorophyll and the oxygen saturation in the deep-water zone very well (within the uncertainty bands of the empirical data). We show that the model, with new calculation routines for macrophytes and clams, also describes the conditions in Ringkøbing Fjord well, which means that the model is a useful general tool for interpretations of changes in coastal ecosystems. The model is simple to apply in practice since all driving variables may be readily accessed from maps or regular monitoring programs.

Linking phytoplankton community composition with juvenile-phase growth in the Northern QuahogMercenaria mercenaria (L.)

This study examined whetherMercenaria mercenaria (L.) (quahog) growth is influenced by variability in phytoplantkon community composition in the waters of Long Island, New York. Field studies conducted during 1999 and 2000 compared juvenile quahog growth and phytoplankton assemblages between West Sayville (WS), an embayment in Great South Bay along Long Island’s south shore where quahog landings have recently declined, and Oyster Bay (OB), an embayment on Long Island’s north shore where quahog landings are still high. Quahogs grew better at OB than WS during both study years. Centric diatoms were typically the dominant phytoplankton species at OB, and pennate diatoms and dinoflagellates characterized WS. At WS, the phytoplankton community consisted of heterotrophic dinoflagellates during a brown tide in 2000 and pennate diatoms afterward. Nanoflagellates were abundant (105–106 cells ml−1) at WS throughout the summer of 2000. Multiple regression analysis revealed a significant effect of site and temperature on individual clam biomass during both years, but brown tide was only significant during 2000. Biomass comparisons of dominant phytoplantkon taxa with laboratory physiology studies showed that 0B, with its abundance of centric diatoms, likely represented a more nutritional diet for quahogs than pennate diatoms, which were abundant at WS. Small flagellates, which were common at WS, may also have been important for sustaining growth during some months. Variability in plankton assemblages between OB and WS likely represented two distinct, diets that were critical influences on clam growth.

Grazing impact of filter-feeding zoobenthos in a Danish fjord

The grazing impact has been evaluated of filter-feeding animals in Odense Fjord (Denmark) that is characterized by large biomass of filter feeding polychaetes ( Nereis diversicolor), clams ( Mya arenaria) and cockles ( Cerastoderma glaucum). The filtration rate of each population ( F tot) was related to the total water volume ( V tot) in the different areas of the fjord and expressed as ‘the grazing impact’ Q= V tot/ F tot. This quantity is related to the half-time for phytoplankton by t 1/2= Q×ln 2, representing the time to reduce the concentration by a factor of 2 in the water column above the filter-feeding populations, assuming efficient vertical mixing. Based on recorded population densities it was found that the potential grazing impact exerted by the three dominating species could be expressed as Q=0.17 d and t 1/2=0.12 d. This indicates a strong filter-feeder control of the phytoplankton although it should be emphasised that the potential grazing impact may only be realised when the filter-feeding zoobenthos is exposed to the whole water column by effective vertical mixing of the water on windy days.

Disturbance effects of stranded kelp on populations of the sandy beach bivalve Donax serra (Röding)

Twelve beaches on the coast west of Cape Agulhas, South Africa, were surveyed to examine the influence of several physical factors, including stranded kelp, on the zonation, density and biomass of populations of the wedge clam Donax serra. Biomass and density of adult clams were significantly higher on beaches with lower cover of stranded kelp; this pattern did not hold for juveniles. Discriminant analysis of beaches with and without Donax classified these beaches 100% correctly — the most important factor being kelp cover followed by average winter temperature, wave height, morphodynamic state, beach length, wave period and beach slope. Beaches with Donax had a significantly lower percentage of the drift line covered by stranded kelp (mean=30%) than beaches without Donax (mean=77%). On two beaches of the southwest coast with similar morphodynamics and temperature regimes but different kelp cover, adult populations were centered in different zones: in the low intertidal to subtidal where kelp cover was higher (mean=42%) and in the mid-intertidal where no kelp was found. A disturbance hypothesis is developed to explain the observed patterns: kelp stranding in varying quantities interferes with Donax feeding and burrowing activities, dislodging animals of increasing size gradually downshore to the saturation and inner surf zones, where adult populations are eventually established. In the presence of extremely high quantities of stranded kelp, both juveniles and adults would be dislodged from the intertidal to the subtidal. The disturbance process may interact with several other physical factors, being magnified when coupled with low temperatures and small waves on reflective pocket beaches with steep slopes and coarse sands. Additionally, kelp gulls predate dislodged animals, explaining the absence of clams in a stretch of 20 m above the swash zone. It is thus concluded that the zonation patterns here observed are the result of a disturbance effect triggered by stranded kelp and mediated by multiple interacting factors. The adaptations of Donax serra that enable it successfully to colonize intertidal and subtidal habitats of the beach ecosystem are discussed.

Substrate Additive Studies for the Development of Hardshell Clam Habitat in Waters of Puget Sound in Washington State: An Analysis of Effects on Recruitment, Growth, and Survival of the Manila Clam, Tapes philippinarum, and on the Species Diversity and Abundance of Existing Benthic Organisms

To meet demands of expanding tribal and recreational hardshell clam fisheries, the Washington State Department of Fish and Wildlife has been developing new hardshell clam habitat. In Oakland Bay, near Shelton, Washington, two artificial substrates were studied to determine the best substrate for use in improving Manila clam (Tapes philippinarum) production. Treatments consisted of a control of natural substrate; a 10 cm layer of nonuniform size gravel ranging from 6 mm to 19 mm in diameter; and a 10 cm layer of a 50∶50 mixture of gravel and crushed oyster shell ranging from 10 mm to 30 mm in diameter. Effects of these substrates on species diversity and relative abundance of existing, benthic organisms were also investigated. Recruitment of Manila clam seed, less than 10 mm shell length, was significantly higher (p<0.05) in the control plots than in the gravel and gravel + shell plots, which showed no significant differences. Recruitment improved in the gravel and gravel + shell plots after a layer of organic debris and fine sediments collected on them. Manila clam survival was significantly higher in gravel and gravel + shell plots compared to control plots. No significant differences in survival were observed between gravel and gravel + shell plots. Manila clam biomass in the gravel and gravel + shell plots was significantly higher than in control plots. Between the gravel and gravel + shell plots there was no significant difference in biomass. Minor differences in species diversity and abundance of existing benthic organisms were observed among treatments. Gammarid amphipods and nemertean worms were enhanced in gravel and gravel + shell plots. Densities of polychaete worms were slightly lower in gravel and gravel + shell plots compared to control plots.

Nitrogen flux in giant clams: size-dependency and relationship to zooxanthellae density and clam biomass in the uptake of dissolved inorganic nitrogen

Tridacnid clams live on coral reefs in the Indo-Pacific, in waters containing low concentrations of inorganic nitrogen. This study examined nitrogen flux in the giant clam Tridacna gigas. Adults and juveniles of this species typically occur with symbiotic dinoflagellates of the genus Symbiodinium sp., often referred to as zooxanthellae, which live in their tissues. Intact clams took up or released dissolved inorganic nitrogen (DIN), with the direction and magnitude of the fluxes dependent on clam size. Non-symbiotic larvae and newly-settled juveniles with few zooxanthellae released ammonium. Larger juveniles and adults depleted ammonium or nitrate from seawater, when offered separately. Rates of uptake of DIN, standardized to either clam wet weight or number of zooxanthellae, were highest in small clams (1 to 5 cm in length), and decreased with increasing clam size (>5 cm). Clams maintained in seawater containing high concentrations of ammonium (ca. 20 μM) for ≥ 1 wk generally released ammonium in the dark and exhibited net uptake in the light. Freshly isolated zooxanthellae (FIZ) from small clams had higher uptake rates than FIZ from larger clams, implying that the latter may be more nitrogen-sufficient than the former. The gradient of nitrogen sufficiency in giant clams is related to zooxanthellae density, with peaks of both algal density and rates of uptake of DIN occurring in small sizes of clams, whose growth also appears to be limited by nitrogen availability.

Are symbiotic algae nutrient deficient?

The evidence for the hypothesis that symbionts in alga−invertebrate associations are nutrient deficient is critically evaluated. It is suggested that the evidence is not consistent with algal symbionts being nutrient deficient, but is consistent with the host regulating its population of symbionts by restricting their supply of nutrients. This distinction is due to the substantial release of photosynthate by symbiotic algae and suggests that there are (at least) two distinct regulatory components involved in the characteristics of nutrient-deficient algal cells.