Impact of resuspended sediment on zooplankton feeding in Lake Waihola, New Zealand

1. Wind‐induced sediment resuspension in shallow lakes affects many physical and biological processes, including food gathering by zooplankton. The effects of suspended sediment on clearance rate were determined for a dominant cladoceran, Daphnia carinata, and calanoid copepod, Boeckella hamata, in Lake Waihola, New Zealand. 2. Animals were incubated at multiple densities for 4 days in lake water containing different amounts of suspended lake sediment. Rates of harvest of major food organisms were determined for each sediment level (turbidity) from changes in net growth rate with grazer density. 3. Daphnia cleared all food organisms 7–40 μm in length at similar rates, but was less efficient in its removal of free bacteria, phytoplankton <7 μm, and large cyanobacterial filaments. Elevation of sediment turbidity from 2 to 10 nephelometric turbidity units (NTU) (63 mg DW L^?1 added sediment) reduced Daphnia clearance of phytoplankton, heterotrophic flagellates and ciliates by 72–100%, and of amoebae and attached bacteria by 21–44%. Further inhibition occurred at higher turbidity. 4. Boeckella hamata removed microzooplankton primarily, rather than phytoplankton. The rate at which it cleared rotifers was reduced by 56% when turbidity was increased from 2.5 to 100 NTU. 5. In the absence of macrozooplankton, algal growth increased with sediment turbidity, suggesting that sediment also inhibits rotifer grazing. 6. As mid‐day turbidity in Lake Waihola is ≥10 NTU about 40% of the time, sediment resuspension may play a major role in moderating energy flow and structuring pelagic communities in this lake.     

The effect of high inorganic seston loads on prey selection by the suspension-feeding bivalve, Atrina zelandica

Suspension-feeders are discriminate feeders, selecting prey by size, shape and food quality, this discriminate feeding behaviour has important consequences for the modelling of growth rates, population dynamics and ecosystem change. When given natural seston, the large pinnid bivalve Atrina zelandica (Gray), fed on picophytoplankton (< 2 μm), larger phytoplankton (2-270 μm) and microzooplankton, but preferentially selected algal species within the 2-20 μm size fraction. Selection for ingestion was based on food quality, with morphotype, carbon content and potential toxicity also being important. Microzooplankton were readily ingested and represented 48% of the available diet in terms of carbon indicating they play an important role in the diet of Atrina. Mussels were subsequently fed a selected cultured algal diet consisting of three different types of 2-20 μm sized phytoplankton to assess differences in prey selection and grazing efficiency. When high inorganic suspended sediment concentrations of 500 mg/l were added in addition to the cultured algal diet this caused Atrina to increase filtration and rejection rates and reduced the efficiency of Atrina to select food in all cases. More importantly, there were changes in the species preferentially selected for ingestion. Our results suggest that as well as reducing feeding efficiency increased suspended sediment concentrations may affect prey selection and therefore have consequences for benthic-pelagic coupling beyond that of reduced removal and deposition rates.     

Ecosystem services transcend boundaries: estuaries provide resource subsidies and influence functional diversity in coastal benthic communities

Background

Estuaries are highly productive ecosystems that can export organic matter to coastal seas (the ‘outwelling hypothesis’). However the role of this food resource subsidy on coastal ecosystem functioning has not been examined.

Methodology/Principal Findings

We investigated the influence of estuarine primary production as a resource subsidy and the influence of estuaries on biodiversity and ecosystem functioning in coastal mollusk-dominated sediment communities. Stable isotope values (δ¹³C, δ¹⁵N) demonstrated that estuarine primary production was exported to the adjacent coast and contributed to secondary production up to 4 km from the estuary mouth. Further, isotope signatures of suspension feeding bivalves on the adjacent coast (Dosinia subrosea) closely mirrored the isotope values of the dominant bivalves inside the estuaries (Austrovenus stutchburyi), indicating utilization of similar organic matter sources. However, the food subsidies varied between estuaries; with estuarine suspended particulate organic matter (SPOM) dominant at Tairua estuary, while seagrass and fringing vegetation detritus was proportionately more important at Whangapoua estuary, with lesser contributions of estuarine SPOM. Distance from the estuary mouth and the size and density of large bivalves (Dosinia spp.) had a significant influence on the composition of biological traits in the coastal macrobenthic communities, signaling the potential influence of these spatial subsidies on ecosystem functioning.

Conclusions/Significance

Our study demonstrated that the locations where ecosystem services like productivity are generated are not necessarily where the services are utilized. Further, we identified indirect positive effects of the nutrient subsidies on biodiversity (the estuarine subsidies influenced the bivalves, which in turn affected the diversity and functional trait composition of the coastal sediment macrofaunal communities). These findings highlight the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystems.     

Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.     

Environmental history and physiological state influence feeding responses of Atrina zelandica to suspended sediment concentrations

Epifaunal suspension-feeding bivalves can play important roles in marine ecosystems affecting macrobenthic communities, benthic boundary layers and benthic-pelagic coupling, not just by their presence but also by any changes in feeding behaviour. While seston quality and quantity have consistently been found to be important influences on the feeding rates of suspension-feeding bivalves, factors stressing individuals are also likely to be important, as they may affect energy-dependent thresholds of response. We postulated that (1) history of seston quantity would affect how suspension feeders deal with increases in total suspended particulates, and (2) high-seston concentrations would affect feeding rates more in individuals whose energy reserves were low after spawning. Three sites were selected for short-term (1 day) feeding experiments on the pinnid bivalve, Atrina zelandica. At one site, the experiment was run pre-and postspawning. Atrina exhibited high rejection of filtered particles (mostly 75% to 100%) and high organic absorption efficiencies (0.9-1) at all seston levels. Strong differences in the response of feeding behaviour to increased seston concentrations were observed between A. zelandica from the different sites, with lesser differences observed between times. The site-specific feeding responses to seston concentrations observed are likely to affect our ability to model responses of A. zelandica to sediment loading and to influence the importance of A. zelandica to benthic-pelagic coupling.     

Effects of silt loading on the feeding and mortality of the mysid Mesopodopsis africana in the St. Lucia Estuary, South Africa

Mesopodopsis africana is an abundant zooplankton species in the currently drought stricken St. Lucia Estuary. The estuary is freshwater-starved, partly because its connection with the large Mfolozi catchment has been discontinued. This is due to the extremely high silt load carried by its waters. A series of in vitro experiments were conducted in May, July, August and December 2006, in order to determine the effect of silt loading on the feeding rate and mortality of M. africana. Mysids were subjected to different sediment concentrations in controlled experiments for a 12-hour period. The turbidity regimes were generated by adding pre-weighed silt concentrations, ranging from 0.03 g/L to 13 g/L, to natural estuarine water. Results in all cases showed a major effect of high silt levels upon food availability, as water chlorophyll losses increased with increased silt concentration. Hindrance in the food collecting abilities appears to have a compounding influence as on two occasions, May and August, ingestion rates decreased with an increase in silt concentration. However, in July, mysid ingestion and clearance rates were higher under increased suspended silt concentrations. In such cases, mysids may be clearing more water under conditions of low food concentration, or alternatively, they may switch to feeding on metazoan prey. In the December experiment, there was no relationship between ingestion rate and silt concentration, possibly because food supplies were sufficient to sustain constant ingestion rates or because mysids increased their clearance rates in the more turbid treatments to compensate for any negative effects of silt. Mysid health also declined, with the mean percentage of survivors decreasing consistently with an increase in silt concentration. Results indicate that the overall effect of increased silt load is negative, and subsequently, increased silt loading in the St. Lucia estuary is expected to have detrimental effects on communities of mysids within the estuary.     

The influence of Corophium volutator abundance on resuspension

Two experiments were performed to test the hypothesis that Corophium volutator affects the turbidity of water in estuaries through active resuspension of sediment. One experiment was done in a flume system under different flow velocities, and one in aquaria. A diatom film developed at the sediment surface in both experiments before Corophium was added. This diatom film was supposed to have a stabilising effect on the sediment. In both experiments, the concentration of suspended solids in the water column increased with the density of Corophium individuals. No effect of flow velocity on suspended solids concentration was found. This indicates that, in our flume experiment, active resuspension by Corophium was more important than physical resuspension, at least at low flow velocity (<20 cm s^-1) and in the presence of a diatom film. The critical erosion threshold decreased with increasing Corophium density in the aquarium experiment, indicating that indirect effects of Corophium grazing may become more important at high levels of bottom shear stress. The implications of our findings for suspended solids concentration in estuarine systems are discussed.     

Restoration experiment of Zostera marina L. in a subtropical coastal lagoon

Zostera marina is the dominant seagrass species in coastal lagoons on the western coast of Baja California Peninsula, and due to its coastal location it is threatened by natural and anthropogenic factors, as is happening in Puerto San Carlos, B.C.S., where a fish cannery unloads its wastewater to the beach. Apparently an extensive intertidal meadow replacement was established by great amounts of green macroalgae. We evaluated the possibility to mitigate the impacts of this cannery with transplants of Z. marina meadow using adult plants. The transplant experiment was made in two different seasons for which two undisturbed donor meadows were chosen: El Cuervo and San Carlitos. The winter one obtained a 30% and in San Carlitos 90% after 13 months and the autumn transplant in San Carlos obtained a 0% of survival after 3 months. The results of these transplant activities were reflected in the shoot density at the end of the experiment (San Carlos was of 482 shoots/m² and San Carlitos of 818 shoots/m²s and agree with the density of the natural meadows. This experiment shows that it is possible to develop a small-scale seagrass restoration as mitigation for Baja California coastal lagoons which are under severe threat for coastal development.     

Laboratory experiments on the effects of variable suspended sediment concentrations on the ecophysiology of the porcelain crab Petrolisthes elongatus (Milne Edwards, 1837)

The porcelain crab Petrolisthes elongatus is a particulate suspension feeding species common to coastal areas of New Zealand (NZ). Consistent with the responses of other suspension feeding species, it is likely to be negatively influenced by elevated suspended sediment concentrations. Laboratory experiments were conducted to quantify the effect of temperature (12 °C, 15 °C and 18 °C) and suspended sediment concentration (total particulate matter (TPM): low < 100 mg L^− 1; medium 100-1000 mg L^− 1; high > 1000 mg L^− 1) on the clearance rate (CR in L h^− 1), oxygen uptake rate (VO₂ in mL h⁻¹), net absorption efficiency (AE), and net energy budget (NEB in J h^− 1) of P. elongatus across a range of sizes. Variation in CR and AE was independent of temperature and of body size, but were significantly different (P < 0.05) at low and medium suspended sediment concentrations compared with high suspended sediment concentrations. CR responded in a non-linear manner to changes in TPM, increasing with TPM up to a maximum value at medium-low concentrations (approximately 250 mg L^− 1) and then decreasing thereafter. CR had almost completely shut down at TPM concentrations of > 1000 mg L^− 1 and at particulate organic matter (POM) concentrations of > 250 mg L^− 1. AE was zero at approximate TPM and POM values of 1200 mg L^− 1 and 300 mg L^− 1, respectively. VO₂ was positively correlated with body size and with temperature, but was independent of TPM. NEB values for P. elongatus were low (approx 110 J g^− 1 h^− 1) at low sediment concentrations, were high (approx 320 J g^− 1 h^− 1) at medium sediment concentrations, and were negative (approx − 114 J g^− 1 h^− 1) at high sediment concentrations. These findings indicate that P. elongatus is likely to be food-limited at sediment concentrations of < 100 mg L^− 1, and severely negatively affected at sediment concentrations of > 1000 mg L^− 1, at least for the duration of such events which may persist for 2-3 days in coastal environments where this crab occurs.     

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (N_org) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the N_org deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 μmol N cm⁻² yr⁻¹. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO ₃ ⁻ reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 μmol N cm⁻² yr⁻¹ if DNRA is the dominant pathway. The response to salinization includes a short-term release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO ₃ ⁻ concentrations and greater SO ₄ ²⁻ concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.     

The effect of macrofauna,meiofauna and microfauna on the degradation of Spartina maritima detritus from a salt marsh area

Decomposition of salt marsh plants results from physical, chemical and biological processes including abiotic and biotic fragmentation, microbial decay and chemical transformation. According to literature data, only a few species have the ability to feed directly on living plant material, so fungi and bacteria seem to be the principal competitors for the organic substrates. Nevertheless, by consuming bacteria, protists and fungi associated to the detritus, macrofauna and meiofauna recycle the incorporated nutrients. Moreover, this nutrient regeneration may be seen as an effective factor in maintaining and stimulating bacterial production. In fact, it is well known that many detritus feeding species have very low assimilation efficiencies. The objective of the present study was to compare the nutrient mass balance of carbon; nitrogen and phosphorus in Spartina maritima covered areas and bare bottom sediment, with and without contribution of macrofauna, meiofauna and microbial populations. Nutrients mass balance was studied taking into account the initial and final nutrient concentrations in the sediment, water and plant material. Faunal activity was measured as a function of remineralised carbon, nitrogen and phosphorus. The experimental set-up included sixteen sub-experiments, which varied with respect to type of fauna, plant biomass and oxic status. Each sub-experiment was performed in small glass containers (3 L) containing about 900 g wwt sediment and 2.5 L estuarine water. Plant material, cut from intact plants, sediment cores and estuarine water were brought from the southern arm of the Mondego estuary (Portugal). The results showed that although the bacterial activity was responsible for the Spartina maritima degradation, the presence of meiofauna and macrofauna significantly enhanced the process. Moreover, the presence of Spartina maritima positively affected the mineralisation of the sediment carbon and nitrogen, especially when the three faunal components were present, and denitrification rates were highest in the presence of the macrofauna and meiofauna. The present study suggests that macrofauna and meiofauna have an important role on the ecosystem nutrient flux and that fauna might function as a sink for excess nutrients, that otherwise could be exported to the coastal waters.     

Import of fine marine sediment in tidal areas

Summary In many estuaries the observed import of fine marine sediment can be explained by the presence of a gravitational circulation, which is driven by the longitudinal density gradient caused by fresh water inflow. Examples are the Thames, the Gironde, Columbia river and Savannah river. With this transport mechanism a turbidity maximum is present near the upstream boundary of the gravitational circulation. Even in vertically well mixed estuaries, where the contribution of gravitational circulation to the residual salt transport is of minor importance, the influence on the residual sediment transport may be considerable.In tidal basins with no river inflow, or with a relatively small river inflow as compared to the average tidal discharge (<Q>²/<Q²>0.01) no significant gravitational circulation is present. Yet a residual sediment transport and a landward increase of the suspended sediment concentration is experimentally observed in this type of tidal basins. However, the turbidity maximum characteristic for inhomogeneous estuaries is absent. In an estuary, sediment may accumulate on the bottom in regions where the shear stress from tidal currents and wind waves does not exceed some critical value. The rate of accumulation depends on the capacity of the current velocity field to transport sediment from the seaboundary to the sedimentation areas. The exchange of estuarine waters and seawater by horizontal circulations or shear diffusion is not responsible for this residual import of sediment. These exchange processes would lead to an export instead of an import of sediment as a consequence of the higher suspended sediment concentration of the estuarine waters.Some thirty years ago, Postma pointed out a class of sediment transport mechanisms which may explain the import of marine sediment in homogeneous tidal basins. These mechanisms are based on the existence of an ebb-flood asymmetry in the cycle of sedimentation, erosion and subsequent tidal displacement of sediment. An ebb-flood asymmetry must be understood here as follows: certain tidal characteristics designated here by f (for example the current velocity, or the bottom shear stress), evaluated in a frame moving with the cross-sectionally averaged tidal velocity, violate the equality f(t)+f(t+1/2 T)=0, where T is the length of the tidal period. The net displacement of sediment due to such an ebb-flood asymmetry is called tide-induced residual sediment transport. The geometrical configuration of a tidal basin determines to a large extent the ebb-flood asymmetry. For example, a strong decrease of the relative bottom depth, which occurs in the landward part of a tidal basin, causes an ebb-flood asymmetry due to which the import of fine sediment exceeds the export.The tide-induced residual import of marine sediment is not sufficient in itself to explain the experimentally observed increase of suspended sediment in the landward part of a tidal basin. An increase of the suspended sediment concentration would be expected there where the capacity of the residual landward transport decreases (cf. the turbidity maximum near the landward limit of gravitational circulation). Generally, however, the capacity of the tide-induced residual transport increases in landward direction, as a result of which the suspended sediment concentration can be expected to decrease. This apparent contradiction can be explained by the action of wind waves, which may, at irregular intervals, bring back into suspension large amounts of sediment which have settled under less severe wave conditions. Wind action may also change the tide-induced residual sediment import into export in the shallow landward part, causing a redistribution of sediment inside the tidal basin. The time that fine marine sediment remains in suspension inside the tidal basin is therefore strongly increased by the action of wind waves.The influence of gravitational circulation on the net import of fine sediment in estuaries is illustrated with field data collected in the Rotterdam Waterway and the western Waddensea. The tide-induced residual sediment transport is shown to be the major transport process in large parts of the eastern Waddensea and the Oosterschelde Basin. It provides a quantitative explanation of the experimentally observed import of fine sediment in the former and export in the latter tidal system.     

Sediment Composition Influences Spatial Variation in the Abundance of Human Pathogen Indicator Bacteria within an Estuarine Environment

Faecal contamination of estuarine and coastal waters can pose a risk to human health, particularly in areas used for shellfish production or recreation. Routine microbiological water quality testing highlights areas of faecal indicator bacteria (FIB) contamination within the water column, but fails to consider the abundance of FIB in sediments, which under certain hydrodynamic conditions can become resuspended. Sediments can enhance the survival of FIB in estuarine environments, but the influence of sediment composition on the ecology and abundance of FIB is poorly understood. To determine the relationship between sediment composition (grain size and organic matter) and the abundance of pathogen indicator bacteria (PIB), sediments were collected from four transverse transects of the Conwy estuary, UK. The abundance of culturable Escherichia coli, total coliforms, enterococci, Campylobacter, Salmonella and Vibrio spp. in sediments was determined in relation to sediment grain size, organic matter content, salinity, depth and temperature. Sediments that contained higher proportions of silt and/or clay and associated organic matter content showed significant positive correlations with the abundance of PIB. Furthermore, the abundance of each bacterial group was positively correlated with the presence of all other groups enumerated. Campylobacter spp. were not isolated from estuarine sediments. Comparisons of the number of culturable E. coli, total coliforms and Vibrio spp. in sediments and the water column revealed that their abundance was 281, 433 and 58-fold greater in sediments (colony forming units (CFU)/100g) when compared with the water column (CFU/100ml), respectively. These data provide important insights into sediment compositions that promote the abundance of PIB in estuarine environments, with important implications for the modelling and prediction of public health risk based on sediment resuspension and transport.     

Suspended sediment alters predator–prey interactions between two coral reef fishes

Sediment derived from agriculture and development increases water turbidity and threatens the health of inshore coral reefs. In this study, we examined whether suspended sediment could change predation patterns through a reduction in visual cues. We measured survivorship of newly settled Chromis atripectoralis exposed to Pseudochromis fuscus, a common predator of juvenile damselfishes, in aquaria with one of four turbidity levels. Increased turbidity led to a nonlinear response in predation patterns. Predator-induced mortality was ~50 % in the control and low turbidity level, but exhibited a substantial increase in the medium level. In the highest turbidity level, predation rates declined to the level seen in the control. These results suggest an imbalance in how the predator and prey cope with turbidity. A turbidity-induced change to the outcome of predator–prey interactions represents a major change to the fundamental processes that regulate fish assemblages.     

Transport of sediment in mangrove swamps

The transport of suspended sediment in mangrove swamps is controlled by three dominant processes. First, the transport processes in the estuaries and coastal waters draining the swamp, including flocculation, tidal pumping, baroclinic circulation, trapping of the smallest particles in the turbidity maximum zone, and the effect of the mangrove tidal prism. Second, the mechanical and chemical reactions in mangrove waters destroying flocs of cohesive sediment in suspension. Third, biological processes have a dominant influence on the ultimate fate of clay particles in mangroves.     

A sediment suspension system for bioassays with small aquatic organisms

Exposure of aquatic organisms to suspended sediments can impair growth and survival and increase bioaccumulation of sediment-associated contaminants. However, evaluation of the effects of suspended sediments and their associated contaminants on aquatic organisms has been hampered by the lack of a practical and inexpensive exposure system for conducting bioassays. We present a cost-effective system for assessing the effects of suspended sediments and associated contaminants on small aquatic organisms. A 7-day suspension test was conducted with nominal sediment concentrations ranging from 0.0 to 5.0 g 1^–1. The system maintained relatively constant suspended sediment concentrations, as measured by turbidity, and caused minimal mortality to test organisms.     

On the importance of sediment-water exchange processes of ammonia to primary production in shallow areas of the Sado estuary (Portugal)

Coupling between the seasonal phytoplankton growth cycle and resuspension mechanisms in low depth areas was studied. Tidal flood in an estuarine inlet with a large area of intertidal flats was investigated with respect to nutrients and suspended particulate matter, during two different periods of the year. A prespring bloom period in March and another during the productive summer term in August were picked. Results indicate that tidal resuspension mechanisms are important in mobilizing NH₄ ⁺ from the sediment surface pool, and that this additional input of the limiting nutrient is rapidly consumed in the flooding water column, in benefit of microphytes resident in the nutrient-poor, incoming,coastal waters.     

The importance of sulfate reduction associated with Ulva lactuca thalli during decomposition: a mesocosm experiment

The decay of the macroalga Ulva lactuca was followed for 54 days in a controlled laboratory experiment. The experiment focused on the activity of sulfate reducers in different compartments (water, thalli and sediment) of the experimental system. In addition to sulfate reduction, the concentrations of sulfide, carbon dioxide, sulfate, carboxylic acids and pH were determined at regular intervals. Interestingly, 90% of the system-integrated sulfate reduction was carried out in the water column by thallus-associated sulfate reducers. The sediment accounted for about 10% of the integrated sulfate reduction activity, while sulfate reduction carried out by free-living sulfate reducers was insignificant and represented less than 1% of total sulfate reduction. Sulfate reduction rates in the water column were below the detection limit at the beginning of the experiment and were detected after 1 week of incubation. Sulfate reduction rates associated with thalli were measurable immediately after the experiment was started and increased very rapidly, reaching extremely high rates after 1 week of incubation. Sediment sulfate reduction rates had increased to twice the initial value by day 30 after which they remained constant. Thallus-associated sulfate reduction rates (SRR) were of the same level in all layers of the algal mat throughout the experiment. Our results indicate that sulfate-reducing bacteria were present on the thalli when the experiment was initiated and that the water column colonization by sulfate-reducing bacteria from the sediment was less important. This would explain the rapid accumulation of hydrogen sulfide in the water column during macroalgal decay events in coastal marine environments.     

The effect of sediment redox potential on nitrogen uptake,anaerobic root respiration and growth of Spartina alterniflora loisel

A system developed for growing plants in estuarine sediment at controlled redox potential (Eh) was used to study the effect of Eh on anaerobic root respiration, uptake of added labelled nitrogen and photosynthetic rates of Spartina alterniflora Loisel. Plant growth, estimated by CO₂ fixation and nitrogen uptake, was not affected by sediment redox potential ranging from oxidized (+500 mV) to strongly reducing (?200 mV). Anaerobic root respiration, evident by alcohol dehydrogenase activity, increased with decreasing sediment redox potential. The results suggest that neither redox potential per se nor anaerobic root respiration can account for the growth differences of S. alterniflora observed in Louisiana's coastal marshes.     

Impacts of Sediments on Coral Energetics: Partitioning the Effects of Turbidity and Settling Particles

Sediment loads have long been known to be deleterious to corals, but the effects of turbidity and settling particles have not previously been partitioned. This study provides a novel approach using inert silicon carbide powder to partition and quantify the mechanical effects of sediment settling versus reduced light under a chronically high sedimentary regime on two turbid water corals commonly found in Singapore (Galaxea fascicularis and Goniopora somaliensis). Coral fragments were evenly distributed among three treatments: an open control (30% ambient PAR), a shaded control (15% ambient PAR) and sediment treatment (15% ambient PAR; 26.4 mg cm⁻² day⁻¹). The rate of photosynthesis and respiration, and the dark-adapted quantum yield were measured once a week for four weeks. By week four, the photosynthesis to respiration ratio (P/R ratio) and the photosynthetic yield (F_v/F_m) had fallen by 14% and 3–17% respectively in the shaded control, contrasting with corals exposed to sediments whose P/R ratio and yield had declined by 21% and 18–34% respectively. The differences in rates between the shaded control and the sediment treatment were attributed to the mechanical effects of sediment deposition. The physiological response to sediment stress differed between species with G. fascicularis experiencing a greater decline in the net photosynthetic yield (13%) than G. somaliensis (9.5%), but a smaller increase in the respiration rates (G. fascicularis = 9.9%, G. somaliensis = 14.2%). These different physiological responses were attributed, in part, to coral morphology and highlighted key physiological processes that drive species distribution along high to low turbidity and depositional gradients.