Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

The Interplay of Macrofauna Functional Groups Shapes Nitrogen Cycling in Oligotrophic Estuarine Sediments

The effects of single macrofauna species on benthic nitrogen (N) cycling has been extensively studied, whereas the effect of macrofauna communities on N-related processes remains poorly explored. In this study, we characterized benthic N-cycling in bioturbated sediments of an oligotrophic northern Baltic waters (Öre estuary). Solute fluxes and N transformations (N2 fixation, denitrification and DNRA) were measured in sediments and in macrofauna-bacteria holobionts to partition the role of three dominant macrofauna taxa (Limnecola balthica, Marenzelleria sp. and Monoporeia affinis) in shaping N-cycling, and to disentangle the contribution of different functional groups within the community. In the studied area, benthic macrofauna comprised a low diversity community with extremely high local dominance of three macrofauna taxa, which are widespread and dominant in the Baltic. The biomass of these three taxa in the benthic community explained up to 30% of variation in measured biogeochemical processes, confirming their role in ecosystem functioning. The results also show that these taxa significantly contributed to the benthic metabolism and N-cycling (direct effect) as well as reworked sediments with positive feedback to dissimilative nitrate reduction (indirect effect). Taken together, these functions promoted a re-use of nutrient at the benthic level, limiting net losses (e.g. denitrification) and effluxes to bottom water. Finally, the detection of multiple N transformations in dominating macrofauna holobionts suggested a community-associated active and versatile microbiome, which alternatively contributes to the biogeochemical processes. The present study highlights hidden and interactive effects among microbes and macrofauna, which should be considered in analysing benthic functioning.

Benthic Metabolism in Fluvial Sediments with Larvae of Lampetra sp

Lampreys spend their larval stage within fine sand fluvial sediments, where they burrow and act as filter feeders. Lamprey larvae (ammocoetes) can significantly affect benthic-pelagic coupling and nutrient cycling in rivers, due to high densities. However, their bioturbation, feeding and excretion activities are still poorly explored. These aspects were investigated by means of laboratory incubations of intact sediments added with ammocoetes and of animals alone. Oxygen respiration, nutrient fluxes and excretion rates were determined. Individual ammocoete incubations suggested that biomass-specific oxygen consumption and ammonium, reactive phosphorus and silica excretion were size-dependent, and greater in small compared to large individuals. The comparison of ammocoetes metabolic rates with rates measured in intact sediments revealed that ammocoetes activity decreases significantly when they are burrowed in sediments. Furthermore, results suggest that a major fraction of ammonium excreted by ammocoetes was assimilated by benthic microbes or microalgae to overcome in situ N-limitation. Alternatively, part of the excreted ammonium was oxidized and denitrified within sediments, as nitrate uptake rather increased along with ammocoetes density. Ammocoetes excreted reactive phosphorus and silica but such production was not apparent in bioturbated sediments, likely due to microbial or microalgal uptake or to immobilization in sediments.

Impacts of bioturbation by Venus clam Cyclina sinensis (Gmelin, 1791) on benthic metabolism and sediment nutrient dynamics in a shrimp-clam polyculture pond

Dynamics of benthic inorganic nutrients are key biogeochemical components of sediment metabolism and ecosystems. This study investigated the roles of the bivalve Cyclina sinensis (Gmelin, 1791) and its influence on benthic metabolism, nutrient fluxes and sediment oxygen consumption (SOC) in a shrimp-clam polyculture system in comparison with shrimp culture ponds without incorporating clams, in Ningbo Zhejiang China. The benthic inorganic nutrients fluxes (ammonium-NH4+, nitrate plus nitrite-NO3-+NO2- and phosphate-PO43-) and SOC were measured across the sediment-water interface with dark incubation experiments. The results showed that there were significantly higher nutrient fluxes from the sediment into the overlying water (p<0.05) in the treatment ponds in the order of NH4+ > PO43- > NO3- + NO2-. The SOC varied significantly (p<0.05) and was three-times higher than that of the control ponds. Water quality (dissolved oxygen, pH, chlorophyll-a and salinity) recorded showed slight variations over time but were not significantly different (p>0.05) between the control and treatment ponds. Sediment organic matter and chlorophyll-a concentration measured in the shrimp-clam ponds varied significantly (p<.05) as compared to control ponds. The results of this study suggest that the bioturbation activities by C. sinensis promoted the SOC, sediment organic matter degradation and mineralisation process that increased the exchange of nutrients and oxygen uptake between the sediment and the overlying water.

Impact of bottom trawling on sediment biogeochemistry: a modelling approach

Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deep penetrating gear (e.g. a tickler chain beam trawl) and a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers, and led to significantly lower amounts of ammonium (43–99 % reduction) and organic carbon in the top 10 cm of the sediment (62–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types, and between trawling frequencies. The shallow penetrating gear had a slightly smaller effect on benthic denitrification than the deep penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generates significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Understanding effects of small dams on benthic metabolism and primary production in temperate forested streams

Dams can alter the chemical and physical conditions of downstream environments by increasing stream temperatures, altering nutrient limitation, reducing flow variability, and reducing fine sediment deposition. However, little is known about how fundamental stream ecosystem processes like productivity and respiration respond to dams. Nutrient diffusing substrates were installed in three dam streams and three control streams to evaluate the effect of dams on benthic gross primary productivity (GPP), respiration (R), and chlorophyll α production. Dam streams were an average of 5.6 °C warmer than control streams but GPP, R and chlorophyll α were not different between control and dam streams. Phosphorus enrichment increased heterotrophic R relative to controls (~1.8×) but not autotrophic GPP, R or chlorophyll α. Stream nutrient concentrations and nutrient limitation of heterotrophic R were similar in dam and control streams, suggesting that the dams had limited effects on nutrient transport downstream. Autotrophic GPP, R and chlorophyll α were limited by light and varied within and across streams, potentially masking our ability to detect differences caused solely by dams. Dams may alter stream ecosystem func- tion but consideration of other factors associated with and independent of dams is critical for predicting ecosystem responses to dams.