Much research on the toxic alga Aureococcus anophagefferens (brown tide) has focused on its effects on bivalve suspension feeding, but less is known about how brown tide influences bivalve growth. This study examined the extent to which background levels, defined as concentrations too low for toxicity to inhibit bivalve feeding, of A. anophagefferens influenced the growth and feeding physiology of northern quahogs Mercenaria mercenaria compared to other phytoplankton common to Long Island, New York, waters. Juvenile quahogs were fed either unialgal cultures of A. anophagefferens, non brown tide species, or diets mixed with background levels of brown tide. Absorption efficiency (AE) was determined using the C-14:Cr-51 dual-tracer method, and growth was determined by overall biomass change. Results showed that unialgal diets resulting in the highest AE, specifically Isochrysis galbana and Thalassiosira pseudonana, were associated with rapid M. mercenaria growth. Conversely, Nitzschia closterium resulted in a comparatively low AE and a loss in quahog biomass. Diets mixed with brown tide resulted in a significantly lower AE than the corresponding unialgal diet for all phytoplankton species except N. closterium. Additionally, mixed diets exerted a small negative influence on quahog growth compared to unialgal diets. These observations suggest that quahogs may suffer subtle, chronic effects when A. anophagefferens is present in the field at background levels. Moreover, the different responses of M. mercenaria to each experimental diet have broad implications for understanding how phytoplankton community composition may influence bivalve growth patterns in the field.

Sediment mixing processes were investigated using inert tracer experiments, benthic macrofaunal community analysis, and surveys of ray feeding pits to quantify the relative rates and controls of physical and biological reworking on Debidue Flat, an intertidal sande at in South Carolina. Sediment reworking on Debidue Flat was rapid, with both advective and biodiffusive mixing operating over different vertical spatial scales. Physical reworking by tidal currents dominated initial transport of the tracer in the top 5‐ 10 cm on timescales of ;30 days. Although the exact mechanism of tracer transport is unclear, it is most likely due to active e uidization of surface sediments during stages of the tide followed by a density-driven settling of tracer resulting in a steady downward transport to the depth of bedform reworking. Biodiffusive mixing was evident throughout the sampled interval (;30 cm) and dominated reworking at depths greater than 10 cm. Estimated biodiffusive mixing coefe cients ( D b) were high all year (0.15‐ 0.28 cm 2 d 2 1 ), and were comparable to values reported for coastal bioturbated muds. The haustoriid amphipod Pseudohaustorius caroliniensis was most likely responsible for tracer dispersal in the 10‐ 30 cm interval based on its distribution, abundance, size, and observed burrow structures. Ray pit excavation and ine lling were seasonal disturbances that contributed ;12‐ 22% to spatially averaged advective transport rates but were locally intense and capable alone of turning over the entire upper ;15 cm of the e at in ;100‐ 1000 d. We propose that the mixing processes on Debidue Flat promote an unconstricted, open sediment matrix that maintains the high permeability required for the rapid porewater exchange to 25 cm noted for this system. Thus, in addition to redistributing organic substrates, physical and biological particle mixing play important roles in controlling permeability of e at deposits and quantie cation of these processes is important to understand controls on permeability and biogeochemical cycling of solutes in sandy systems.

Study of the flux and fate of reactive organic material (OM) within Debidue Flat, an intertidal sandflat in the North Inlet estuary, South Carolina, demonstrated that this coarse‐grained deposit is a dynamic, open system that experiences rapid OM decomposition and exchange of solutes in the top 30 cm of the sediment column. The fluxes of reactive OM through Debidue Flat were high during all seasons (27–170 mmol C m2 d-1) and were comparable to fluxes in muddy portions of the North Inlet estuary. Porewater decomposition products were N‐ and P‐rich, the modeled reactivity of organic carbon undergoing decomposition was high (first‐order rate constant, k = 0.02 d-1), and abundant extractable chlorophyll a was measured year‐round; all properties were consistent with marine algal derived substrates. Porewater solute profiles were controlled by advective flow that rapidly exchanged porewater with overlying waters to ~25 cm depth on timescales of hours. Thus, these sandflats act like an unsteady “trickling bed filter,” capturing or generating reactive organic particles, rapidly remineralizing OM, and recycling nutrients. Macrobiological structures within the flat altered the amounts and reaction rates of OM on various spatial and temporal scales. Relatively elevated OM decay rates were associated with the burrows of Callichirus major, a deepburrowing thalassinid shrimp. Large stingray feeding pits accumulated fine grained OM, locally clogging the “trickling bed filter” and inhibiting porewater advection. As illustrated by Debidue Flat, intertidal sands can be sites of high OM flux and turnover and play an important role in biogeochemical cycling in estuarine systems.

We investigated the hypothesis that absorption efficiencies for organic matter (OM) and hydrophobic organic contaminants (HOC) correlate with body size in the deposit feeding polychaete Nereis succinea (Frey and Leuckart). Gut passage time (GPT) in N. succinea is approximately five to ten times shorter in juveniles than in adults. Since shorter GPT is likely to diminish the efficiency of intestinal digestion and solubilization, one would expect juvenile worms to have significantly diminished absorption efficiencies compared to adults. To test this hypothesis, we measured absorption efficiencies (AE) for radioactively labeled phytoplankton (Pseudonitzschia sp.), sediment OM, and three sediment-bound hydrophobic organic contaminants (tetrachlorobiphenyl [TCBP], hexachlorobenzene [HCB] and benzo(a)pyrene [BaP]) over a gradient of body size spanning 10–110 mm. We furthermore measured gut pH and gut fluid surface tension (surfactancy) in relation to body size, to examine whether small worms might compensate for shorter GPT by having comparatively more aggressive gut conditions. Absorption efficiencies were measured in pulse-chase feeding experiments over 5–48 h. Live phytoplankton was absorbed with AEs of 55–95%, while bulk sediment OM was absorbed with AEs of only 5–18%. Sediment-bound TCBP, HCB and BaP were absorbed with AEs of 55–92%. AEs were commonly higher in larger worms, and linear and Ivlev-type regressions of AE onto body size explained more than 59% of the AE variance in any treatment. AEs for phytoplankton and OM correlated strongly with depuration time, i.e. the time until first detection of non-radioactive “chaser”-feces. In contrast, no time dependency of AE was detected for HOCs, albeit over a narrower range of depuration times. Gut fluid pH of N. succinea ranged between pH 5.8 and 7.7, and was on average slightly higher (closer to seawater pH) in larger individuals. Surface tension, measured as drop contact angle, was greatly diminished relative to seawater in all worms, with greatest differences found in large worms. In contrast to AE measurements, only 10–20% of the variance in gut surfactancy and pH data was explained by body size, suggesting that differences in gut chemistry (pH and surfactancy) play a subordinate role in explaining the higher AEs in adult worms. We conclude that gut chemistry is likely to set upper and lower limits on absorption efficiency of food and sediment-bound HOCs in N. succinea, while body size, and in the case of food uptake, differences in GPT, probably account for most of the variance within this range.

The digestive tracts of deposit-feeding organisms constitute diagenetic microenvironments whose biogeochemical conditions may differ considerably from those in the surrounding sediment. To quantify the chemical properties inside the intestines of small (< 2 cm) deposit-feeding polychaetes, we developed protocols for measuring pH, redox potential (Eh), and extracellular protease activity using noninvasive, microfluorimetric and colorimetric techniques. Juvenile Nereis succinea were used as test species. We report here preliminary observations on the gutpH, Eh, and protease activity of N. succinea and on gut pH of three other deposit-feeding worms, Streblospio benedicti, Polydora cornuta and Eygospio sp. A method to determine gut surfactancy, currently under development, is also discussed. Measurements were performed by feeding reactive tracers to animals and using video microscopy and image analysis to quantify changes in fluorescent or colorimetric properties inside their intestines. Gut pH was measured by the pH-indicator fluorescein. All worms analyzed exhibited moderately acidic to neutral mid guts (pH 5.5-7.5), with a gut pH consistently below the pH of seawater (pH 8). Gut redox of N. succinea was estimated by feeding the worms tetrazolium salts of decreasing reducibility and analyzing their feces for the presence of colored formazans. Large LV. succinea were capable of reducing neotetrazolium (NT, Eh = -170 m V) and small individuals were only capable of consistently reducing iodonitrotetrazolium violet (INT, Eh = -90 mV). Gut protease activity oft. succinea quantified by the hydrolysis of ingested BODIPY-FL labeled casein over time, was pronounced and appeared to encounter substrate limitation within a few minutes after ingestion, which seemed to elicit renewed ingestion activity. The in vivo methods described here may be expandable to a broader range of ecophysiological questions concerned with the digestive capabilities of animals and how these may be influenced by ontogenetic and environmental factors.

Solubilization of sediment-bound hydrophobic contaminants (HOCs) by gut fluids of deposit-feeding polychaetes greatly exceeds solubilization by seawater. We present evidence that digestive surfactants exert a central role in HOC desorption, and that the degree of in vitro solubilization by gut fluids is an excellent predictor of HOC absorption efficiency (AE) by the respective worm species. We compared in vitro solubilization of sediment-bound C-14-hexachlorobenzene (HCB) and C-14-tetrachlorobiphenyl (TCBP) by gut fluids of 2 deposit-feeding polychaete species, Nereis (Neanthes) succinea and Pectinaria (Cistenides) gouldii, to AEs measured in Live worms by pulse-chase methodology. N, succinea desorbed 72 % HCB and 79 % TCBP in vitro (during 6 h incubations), and absorbed both compounds with 73 % efficiency, while P. gouldii desorbed only 37 % HCB in 6 h, and analogously absorbed only 37 % HCB. Higher desorption and absorption efficiencies of N. succinea were accompanied by greater gut-fluid surfactancy and higher micelle concentration (determined by drop contact angle) compared to P, gouldii. Calibration of desorption efficiencies with a synthetic surfactant, sodium dodecyl sulfate (SDS), showed that N. succinea gut fluid desorbed a similar amount of HOC as a 1 % (ca 3.5 mM) SDS solution, whereas P, gouldii gut fluid was equivalent to a 0.25 % (ca 0.9 mM) SDS solution. Detailed analysis of the kinetics of HOC desorption (after 1, 45 and 360 min) showed that gut fluids from both polychaetes desorbed more than two-thirds of the bioavailable HOC within the first minute, suggesting that digestive desorption occurs rapidly and that gut-residence time has only minor influence on the degree of desorption or absorption of sediment-bound HOCs.

It has been demonstrated that the deposit-feeding oligochaete Limnodrilus hoffmeisteri inhabiting Foundry Cove (FC), a severely cadmium (Cd)-contaminated cove located on the Hudson River, New York, USA, has evolved resistance to Cd. In this study we investigate how this resistance influences Cd trophic transfer from this oligochaete to the grass shri.mp Palaernonetes p u g ~ o . Cadmium-resistant worms collected from FC and nonresistant worms collected from an adjacent unpolluted site were investigated for differences in Cd tolerance, accumulation, subcellular distribution and bioavailability to shrimp. FC worms were more tolerant of Cd, surviving twice as long as worms from the unpolluted site during a toxicity bioassay The 7 d concentration factor of Cd-resistant worms was 4 times greater than that of nonresistant worms (2020 vs 577). There were also differences between worm populations with respect to subc~llular Cd distributions. Cd-resistant worms produced metallothionein-like proteins (MT) as well as metal-nch granules (MRG) for Cd storage and detoxification; nonresistant worms only produced MT These differences in subcellular Cd distributions led to large differences in Cd bioavailability to shrimp; shrimp fed Cd-resistant worms absorbed 21 % of the ingested Cd, while those fed nonresistant worms absorbed roughly 4 times that amount (-757:,). These absorption efficiencies were in good agreement with the proportions of Cd bound to the worm's most biologically available subcellular fractions (i e , the cytosol and organelles). Although Cd-resistant worms predominantly stored the toxic metal In biologically unavailable MRG, their increased accumulation of Cd would still result in substantial trophic transfer to shrimp because of the storage of Cd in the biologically available fractions. This work demonstrates that the evolution of Cd resistance can have profound implications for Cd bioavailability and cycling within aquatic ecosystems.

The deposit-feeding oligochaete Limnodrilus hoffmeisteri possesses metallothionein-like proteins and metal-rich granules for storing and detoxifying cadmium (Cd). in this study we investigated the bioavailability of Cd sequestered within this oligochaete by conducting feeding experiments with Cd-109-labeled oligochaetes and the omnivorous grass shrimp Palaemonetes pugio. We also make predictions on Cd trophic transfer based on oligochaete subcellular Cd distributions and absorption efficiencies of Cd by shrimp. Cytosol (including metallothionein-like proteins and other proteins) and a debris fraction (including metal-rich granules and tissue fragments) isolated from homogenized Cd-109-labeled oligochaetes were embedded in gelatin and fed to shrimp. The Cd-109 absorption efficiencies of shrimp fed these subcellular fractions were 84.8 and 48.6%, respectively, and were significantly different (p < 0.001), indicating that Cd-109 bound in these fractions was not equally available to a predator. Mass balance equations demonstrate that shrimp fed whole worms absorb 61.5% of the ingested Cd-109, an absorption efficiency similar to that obtained experimentally (57.1%). Furthermore, the majority of the absorbed Cd-109 comes from the fraction containing metallothionein-like proteins (i.e. cytosol). Cd-109 absorbed from the debris fraction probably comes from the digestion of tissue fragments, rather than metal-rich granules. The ecological significance of these findings is that prey detoxification mechanisms may mediate the bioreduction or bioaccumulation of toxic metals along food chains by altering metal bioavailability. Another important finding is that trophic transfer of metal can be predicted based on the subcellular metal distribution of prey.

Functional group analysis was used to determine the major bioturbators in the Dry Tortugas, Florida Keys. The surface community is dominated by surface deposit feeding polychaetes and burrowing bivalves capable of mixing the top 0–4 cm of the sediment on time scales of days to weeks. Bioturbation by the Notomastus sp. and Callianassa sp. deep community effectively removes primary sedimentary structures. Their fossilized burrow structures may be geologically important in this system. Surface microtopography is controlled by stabilizing and destabilizing forces that determine the potential for surface resuspension and sediment transport in the Dry Tortugas.

The goal of this study was to relate the distribution and abundance of functional groups of benthic macrofauna to the depth of bioturbation in Eckernförde Bay, Germany. Particle bioturbation was limited to the top 0.5–1.0 cm throughout Eckernförde Bay and is consistent with sedimentological and radiochemical studies. Our results indicate that vertical particle displacement between ingestion and defecation controls the depth of bioturbation and is directly related to the functional group. The benthic community is maintained at a low level of complexity due to a regular disturbance, most likely seasonal hypoxia/anoxia.

Microbial cells in natural environments are often encased in different types of exopolymer secretions (EPS), ranging from tight capsules surrounding individual cells to the looser slime matrices of biofilms. The different physical and chemical properties of exopolymers could have secondary effects on trophic interactions between microbial cells and consumer animals. Laboratory studies showed that capsule EPS is significantly less digestible to consumers than slime EPS, even when extracted from the same bacterial strain. Bacterial cells with EPS capsules are less efficiently digested than noncapsuled cells, suggesting that capsules protect against digestion. Follow‐up experiments determined that polysaccharide‐rich fractions of slime EPS are absorbed with very high efficiencies while protein portions, which are more abundant in capsular polymers, are absorbed relatively poorly. Another series of experiments showed that dissolved organic matter (DOM), when adsorbed directly to the mineralogical portions of sediment particles, is available to deposit feeders. However, the further presence of an exopolymer coating on sediments more than doubled the bioavailability of adsorbed DOM to the consumer. Observations using cold‐stage scanning electron microscopy indicated that exopolymer microenvironments are a common feature of natural marine sediments. Microbial exopolymers range from easily digestible carbon sources to relatively refractory ones that effectively protect some microbial cells from consumer digestion. Exopolymer microenvironments may also make recently adsorbed DOM highly accessible to particle‐ingesting animals.

Laboratory experiments were conducted to determine the influence of changes in sedimentary organic matter and bacteria on ingestion and absorption of a deposit-feeding protobranch bivalve, Nucula proxima.The largest detrital component appears to track the organic matter available for N. proxima and influence its physiological energetic budget. The animal absorbed bacteria more efficiently than detritus, but obtained less organic carbon from the bacteria. Changes in bacterial abundance were related to water temperature, but the absorption efficiency did not track bacterial abundance. Differences in ingestion rate could not be detected between 12 and 25-degrees-C, but temperatures below 12-degrees-C did decrease the pellet production rate of the animal.

This study investigates the separate and interactive effects of food concentration, environmental oxygen level, and body size on the growth rates of Capitella species 1 individuals. Reduction of environmental pO2 or food concentration decreased the volume‐specific growth rates of both large (>1.3 mm3; >1.6 mg wet wt, WW) and small (<1.0 mm3; <1.1 mg WW) worms. Within the high‐food treatments a reduction in environmental pO2 from ∼130 to 35 mm of Hg decreased the growth rates of worms by up to 36% d− 1.Growth of large worms decreased in response to reductions in either pO2 or food concentration. The growth rates of small worms decreased significantly only in response to reductions in environmental pO2 levels. Reductions in either food concentration or environmental oxygen level alone decreased the growth of large worms to the same extent as a concomitant decrease in both. When worms were fed 100% natural sediment, a reduction in average O2 tension from 38 to 21 mm of Hg decreased the growth of large worms from 2% d−1 to −25% d−1. The growth rates of small individuals were unaffected (−4 to −3% d−1).We predict that under conditions of near‐bottom hypoxia or food limitation (e.g. complete pelletization of ingestible sediment) populations of Capitella sp. 1 will experience size‐dependent growth with large animals (>1.6 mg WW) experiencing the greatest decline in growth rate.

Repeated measurements of individual worms over a 28-d period were used to calculated volumespecific growth (G) and egestion rates at a high, constant food ration (> 1000 mg N · m −2 · d−1). Worm sizes ranged from 0.1–9.0 mm3 body volume (BV) (≈20–1600 μg dry weight (DW)), 1.8–18.7 mm live body length). The relationship between G and BV was highly variable but displayed a plateau at sizes between 0.5–3.5 mm3 (120–830 μg D W) where G remained in the range of 18–20% · d−1. In contrast to predictions of empirical growth models, G did not show a monotonie decrease with increasing worm volume.

Measurements of 14C loss from uniformly labeled Hydrobia truncata indicated an average energetic cost in this deposit-feeder of 1.7% of total body carbon · day−1. ≈40–50% of carbon lost was in particulate form, while the remaining 50–60% consisted of dissolved carbon (dissolved excretory products and CO2). Sediment particle size had no effect on total carbon loss or on the relative fractions of carbon lost in dissolved vs. particulate form. H. truncata from a sandy beach population had a higher particulate carbon loss rate and lower dissolved carbon loss rate than conspecifics from a muddy salt-marsh population. Total carbon loss did not differ between the two groups. Differences in net carbon intake by H. truncata in response to sediment particle size do not appear to be responsible for size and density distributions of field populations.

The effect of sediment particle size on carbon intake by the deposit-feeding gastropod Hydrobia truncata was investigated. Radiotracer techniques were employed to estimate ingestion rate and absorption efficiency of microalgal carbon and sediment processing rate on two size fractions of natural sediment (silt-clay), < 63 μm; medium sand, 300–600 μm). In two of three experiments, ingestion and absorption rates of microalgal carbon were higher on sand, even though microalgal concentrations were 5 and 17 times lower than on silt-clay. Within each sediment type, ingestion and absorption rates were positively related to microalgal abundance, whereas sediment processing rate and absorption efficiency varied little over a 3–4 fold range in microalgal concentration. Examination of the effect of sediment particle size on carbon intake suggests that the inverse relationship between body size and density of hydrobiid populations and sediment grade in nature is not due to a lack of food associated with coarse-grained sediments.

There are many striking similarities between the benthic macrofauna inhabiting marine and lacustrine sediments. Most of the same trophic/functional groups are well represented in both habitats. There is no obvious difference between the types of particulate food sources available for microphagous animals. Temperate lakes and neritic environments support a similar standing stock of macrofauna, which is a function of similar detrital input to the benthos. There is no characteristic difference in P : B ratios between marine and freshwater macrobenthos. Predation and competition have similar important effects on community structure. Likewise, community succession appears to follow the same pattern.There are certain differences between marine and freshwater macrobenthos, however, that appear to relate to fundamental habitat differences. The major difference is that low salinity and the closed, ephemeral nature of most lakes have resulted in little taxonomic similarity between the two faunas. Tentaculate deposit feeders are found only in marine sediments, and interstitial suspension feeders are common only in freshwater muds. The ability to use dissolved organic matter is well developed only in marine animals, which is a consequence of the osmotic problems faced by freshwater animals. The ability of many limnetic species to survive prolonged anoxia relates to the lack of tidal mixing in lakes.