A benthic annular flume for both laboratory and in situ deployment on intertidal mudflats is described. The flume provides a means of quantifying material flux (i. e., biodeposition of suspended ...particulates, sediment resuspension, nutrients, oxygen, and contaminants) across the sediment-water interface in relation to changes in current velocity and benthic community structure and/or population density of key macrofauna species. Flume experiments have investigated the impact of the infaunal bivalve Macoma balthica and the epifaunal bivalve Mytilus edulis on seston and sediment flux at the sediment-water interface. The bioturbator Macoma was found to increase the sediment resuspension and/or erodability by 4-fold, at densities similar to those recorded at the Skeffling mudflat (Humber estuary) (i. e., $>1000\ \text{individuals}\ {\rm m}^{-2}$). There was a significant correlation between sediment resuspension and Macoma density (r = 0.99; p < 0.001), which supported previous in situ field observations indicating bioturbation by Macoma enhanced sediment erodability. Biodeposition rates ( g m-2 h1) of Mytilus edulis and Cerastoderma edule were quantified and related to changes in population density in a mussel bed (Cleethorpes, Humber estuary). Biodeposition rates were up to 40-times the natural sedimentation rates. At the highest mussel bed densities (i. e., 50-100% cover or $>1400\ \text{mussels}\ {\rm m}^{-2}$) the physical presence of this epifaunal bivalve on the sediment surface reduced erosion by 10-fold. The shift from net biodeposition to net erosion occurred at current velocities of 20-25 cm s-1. These results demonstrate that infaunal and epifaunal bivalves can have a significant impact on seston flux or sediment deposition and on sediment resuspension or erodability in estuaries where there are extensive mudflats.
Water renewal in semi-enclosed coastal areas is crucial for the supply of oxygen and seston and for the removal of organic loadings from finfish or shellfish aquaculture sites. Water renewal depends ...on hydrodynamic processes and can have a complex spatial distribution due to irregular topographic features. This study describes some physical oceanography observations gathered in the Richibucto estuary, New Brunswick, Canada, and provides an estimate of the spatial distribution of water renewal in the North Arm, a location in the estuary where the largest American oyster (Crassostrea virginica) aquaculture operation in eastern Canada is located. The estuary changes from a well mixed estuary to a partially stratified estuary depending on runoff conditions. Tides are mixed but mainly diurnal due to the nearby presence of the second M2amphidromic point in the Gulf of St. Lawrence. Tidal amplitudes vary from 0.3 to 0.6 m and show a slight increase some 35 km upstream. Currents in the main channel can reach over 0.60 m s-1during ebb and 0.3 m s-1during flood, with a slack water period of approximately 8 h. Low frequency sea level fluctuations have a range of 0.5 m at the mouth and are coherent within the estuary. Hydrodynamic and advection-dispersion models are used to calculate the spatial distribution of the local renewal time (LRT) in the North Arm for high and low freshwater discharge conditions, using the dissolved tracer method. Results show that the LRT varies from less than 5 d at the downstream end of the North Arm to over 100 d further upstream. When averaged over the entire North Arm, the integral renewal time (IRT) is estimated to vary only from 8 to 21 d depending on the season. The LRT and IRT estimates are major improvements over conventional renewal estimates using tidal prism methods.
Food sources for the deposit-feeding gastropodsAssiminea japonicaandAngustassiminea castanea(Gastropoda: Assimineidae) were estimated using stable carbon and nitrogen isotope ratios. We collected ...animals and potential food materials in reed marshes of the Nanakita River estuary, the eastern part of Miyagi Prefecture, Japan. Feeding experiments were also conducted to confirm whether snails assimilate 4 types of diets. The stable carbon isotope ratios of both assimineid species (mean ± 1 SD:A. japonica, –20.7 ± 0.3‰;A. castanea, –19.8 ± 0.5‰) were closer to that of deposited organic matter from lagoon water (–20.7 ± 0.3‰) than to those of reed litter (–25.4 ± 0.1‰) and the surface soil of the reed marsh (–26.3 ± 0.1‰). The snails that were fed deposited organic matter showed δ13C values similar to the control animals before feeding experiments for both species. The δ13C values of the snails fed litter or soil diet, however, revealed that these snails were able to assimilate organic matter from reed detritus under laboratory conditions. These findings suggest that the salt marsh snails utilized mainly deposited organic matter from lagoon water in the field. Microalgae such as phytoplankton and benthic diatoms in deposited matter are considered to be important food sources for 2 species of assimineids inhabiting salt marshes of the Nanakita River estuary.
Lake Nordbytjernet (Southeast Norway) had up to 40 mg liter-1 dissolved iron and 67 mg liter-1 dissolved manganese in a stagnant layer. The ratio between iron and manganese loading was around 3: 1, ...while the inventory of manganese was six times higher than for iron. High concentrations of dissolved iron occurred solely under strictly anoxic conditions, while dissolved manganese was unaffected by intrusion of oxygen during the autumnal partial circulation. Mixed potentials caused by oxidation of ferrous iron and reduction of manganic oxide or O2controlled the redox potential values, while dissolved manganese appeared to have no influence. The following main processes controlled the sedimentation rates: fluvial supply of suspended material, precipitation of ferric hydroxide and manganese oxide from the water column, and plankton production. Sorption to ferric hydroxide in the oxic zone and reductive desorption in the anoxic zone controlled the deep water stratification of phosphorus, dissolved organic carbon, and some major cations. No similar effects were seen for manganese sedimentation. The iron sedimentation rate was enhanced in the anoxic zone due to redox coupling of iron oxidation and manganese reduction. Phosphorus was depleted in the anoxic zone during periods of substantial ferric hydroxide sedimentation. Sedimentation of manganese under anoxic conditions seemed to be governed by precipitation of carbonate and phosphate. Residence times in the lake were 0.2 yr for iron and phosphorous vs. 3.3 yr for manganese and 1.4 yr for water.
Time-series of hourly clearance, ingestion and absorption rates and absorption efficiency were measured over 48 h for adult sea scallops (
Placopecten magellanicus) held in situ in a coastal ...embayment in Nova Scotia, Canada, during a wind-induced resuspension event. Temporal variations in oceanographic variables, and seston quantity and composition (organic matter, organic carbon, nitrogen, chlorophyll
a, and inorganic particle size spectra) were monitored during the study with moored instruments and hourly water sampling. Resuspension of bottom materials during the storm resulted in large changes in the amount (1 to 30 mg l
−1 total particulate matter) and nutritional quality (25 to 50% organic content) of seston. High sedimentation rates after the storm were accelerated by flocculation, resulting in the rapid settling of resuspended particles and an increase in seston quality. Observed short-term (hourly) fluctuations in clearance rate were not related to storm- or tide-induced changes in seston characteristics but were directly related to flow velocity. Significantly lower clearance rates were observed at relatively low (<4 cm s
−1) and high (>9 cm s
−1) flow speeds. The overall reduction in ingestion rates after the storm resulted from decreased food availability. Hourly absorption efficiency (AE) measurements were closely related to seston quality (total organic, organic C and N content) and AE declined exponentially with decreasing seston quality. Reductions in AE during the resuspension event were offset by the increased ingestion rate, resulting in no significant changes in absorption rates for organic matter, C, or N over the sampling period. As the low food quality of the resuspended matter was balanced by increased availability, any physiological regulation of food acquisition (i.e. clearance rate regulation) would have been irrelevant to maintaining food intake constant.
Concentrations of dissolved and particulate nutrients, chlorophyll, and microorganisms (0.01 to 200 μm) were simultaneously measured during a 1 d survey of 12 stations in Florida Bay, USA, to ...characterize the microbial plankton community with respect to resource limitation. Three distinct types of trophic conditions, reflected in seston elemental stoichiometry and community structure, were identified within the bay. The first type, characteristic of the isolated eastern region, had low nutrient concentrations, imbalanced stoichiometry, and small microbial biomass with a large proportion of bacteria. The microbial community in this region was characterized by weak relationships between microzooplankton and phytoplankton and the predominance of mixotrophic taxa and the autotrophic ciliate Mesodinium rubrum. The second type, found in the north-central region influenced by Taylor Slough inflow, had elevated nutrient concentrations, elemental stoichiometry skewed toward N, and high turbidity. Under these conditions, the picocyanobacterium Synechococcus formed a dense bloom and coincided with an abundant, multi-step microbial food web. Finally, at the boundary with the Gulf of Mexico, low concentrations of nutrients were balanced at approximately the Redfield ratio and supported nanophytoplankton that were tightly correlated with microzooplankton. These data are consistent with the notion of P limitation in Florida Bay but also demonstrate that Si, light, and N may be colimiting to phytoplankton in the eastern, north-central, and western boundary regions, respectively. Our findings suggest that multiple resource gradients, in conjunction with microbial food web processes, are important factors determining the plankton community structure in Florida Bay and should be considered in studies on ecological disturbances.
Seasonal variation in the relative contributions of littoral and pelagic food sources to the diets of open-water zooplankton and subsequent changes in their trophic positions were investigated with ...carbon and nitrogen Stable Isotope Analysis (SIA). We selected three open water stations as truly pelagic, but also influenced by littoral and riverine carbon sources. During each of the four seasons, integrated pelagic zooplankton samples were collected over 0-50 m depth intervals at each site along with seston in the size range 1.2-76 μm. In addition, vertical temperature profiles were measured. Littoral benthos from three sites along the main longitudinal axis of the lake was sampled to serve as a reference for tracing Lago Maggiore's littoral carbon isotopic signature. Among stations differences in δ13C and δ15N signatures of the different components of the pelagic food web, from seston to predatory zooplankton, were statistically non significant, thus confirming that allochthonous input may become important only after exceptional rainfall events. Changes in the δ13C pelagic baseline mirrored mean water temperature (0-50 m) seasonal changes. Similarly to Lake Geneva, they were likely driven by changes in carbon sources for phytoplankton growth during stratification and vertical water mixing. Differently from what observed for the other taxa, the role of littoral food sources was far from negligible (>50%) for diaptomids during winter and spring. We do not know however, whether such a result could be at least partially attributed to the heavy infestation by algal epibionts, or was consequent to the fact that these zooplankters may carry littoral carbon to the pelagial via horizontal migration. In winter, Bythotrephes longimanus was able to prey on Cyclops, thus occupying a trophic position comparable to that of planktivorous fish. Such a result confirms an ability of this visual, invertebrate predator to compete with young zooplanktivorous fish for food resources, similarly to what observed in lakes it recently invaded.
A process-oriented modelling study is used to examine biophysical control of the distribution of particulate organic matter, or seston, in a tidal embayment with shellfish aquaculture. The focus is ...on the spatio-temporal dynamics of seston as influenced by the processes of water motion and mixing, internal primary production of seston, and the clearance of the water volume by the grazing activity of a large bivalve population. A fluid dynamical framework is used wherein seston is treated as a non-conservative tracer in an advection–diffusion equation with additional source and sink terms. An idealized one-dimensional (1D) tidal inlet is first used to examine the sensitivity of tidally averaged seston concentration and flux to variations in tidal transport, internal production, and shellfish grazing. This model is then applied to Tracadie Bay, a tidal inlet off Canada's east coast, to illustrate temporal variability in seston level and flux for a more complex tidal regime. The results of this study suggest that seston flux is mainly under physical control, with its spatial distribution set by tidal transport processes. Seston level, on the other hand, is affected by both grazing and production, with the magnitude of these effects being spatially dependent as dictated by the tidal currents. Grazing and production effects on seston are most pronounced near the head of the inlet, which depends on internal, or local, processes. More seaward areas are buffered against these changes due the advection of seston from the adjacent open ocean. Variation in the spatial distribution of grazing activity demonstrates how local processes have inlet-wide effects. The temporal response of the inlet to tidal changes in the incoming far-field seston flux resembles a low-pass filter with a phase lag; temporal changes in seston at the head of the inlet are highly dampened and occur later than the forcing flux at the mouth. The implications of these results for marine bivalve aquaculture in terms of growth potential (seston level) and carrying capacity (seston flux) are discussed.
The ability to feed on suspended and dissolved organic nutrients may have been retained in predatory gastropods during evolution. The carnivorous muricid neogastropod
Thais clavigera feeds on prey by ...boring through their shells, followed by extracellular digestion and suction of the nutrient-rich fluid of the prey's body tissues. This study reports on the effect of feeding on suspended and soluble organic nutrients (SSONs) on the survival, growth, and various physiological activities including scope for growth and glycogen stores of
T. clavigera. Juvenile
T. clavigera of similar shell length (23.8±1.7 mm) were either starved, fed with mussel
Septifer virgatus, fed with SSONs from homogenized mussel flesh (
S. virgatus), or fed with both mussels and SSONs, and kept in artificial seawater (salinity: 30‰) for 50 days. Ingestion of SSONs by the animals was significant. Feeding with the “soup” (i.e., SSONs) reduced tissue wastage and improved condition index of the snails.
T. clavigera fed in this manner were intermediates between the starved and the mussel-fed groups in terms of mortality, growth, food consumption, respiration, scope for growth, and glycogen content measurements. Furthermore,
T. clavigera fed with both mussels and SSONs exhibited an identical energy requirement and similar values of various physiological measurements as that of those fed solely on mussel flesh. Feeding of SSONs contributed >10% of the overall energy requirement when both SSONs and mussel prey were available. The results indicate that energy from suspended and dissolved organic nutrients can contribute to the maximization of energy input in
T. clavigera, which may favor better survivorship and thus lifetime fitness.
The effect of the chemical composition of seston on the egg-production rates (Er: eggs female−1 d−1) of the copepods Acartia hudsonica Pinhey and Temora longicornis (Müller) was studied in Long ...Island Sound, USA, (41° 00′ N, 73° 05′ W) during spring 1990. The seston was analyzed for carbon, nitrogen, protein, carbohydrate (CHO) and fatty acid content as well as chlorophyll (chl) and ciliate concentrations. Principal component analysis (PCA) revealed significant correlations score >0.7 on a same principal component (PC) between in situ Er of the 2 copepod species and the chemical composition of the seston. Protein, CHO and specific fatty acids correlated strongly with Er of A. hudsonica (CHO negatively). The same components were moderately correlated (score >0.6 on a same PC) with the Er of T. longicornis. The fatty acid requirements of A. hudsonica and T. longicornis were for high ω3:ω6 ratios and low 20:5 to 22:6 ratios. The fatty acid 22:6ω3 was also correlated with Er of both species. The concentration of ciliates and the C:N ratio of seston did not affect Er of any of the species under any condition. Path analysis models were composed to evaluate the important mechanisms controlling the Er observed in this study. The analysis demonstrated the strength and importance of indirect relationships that were not apparent from conventional correlation statistics. The results from 4 path analysis models showed that chl and ciliates exert an important control on natural egg production rates, through their chemical composition, despite the fact that linear correlations between phytoplankton and ciliates and Er were not significant.
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