How are coastal benthos fed?

Water movement can influence the distribution of benthos, in part, by increasing food delivery; however, the impact of advective transport and turbulent diffusion on organic matter flux to nearshore benthic communities is not well quantified. In this study, we measured the vertical particulate organic carbon (POC) and particulate phosphorus (PP) flux in nearshore Lake Michigan using two naturally occurring daughter/parent radionuclide pairs (234Th/238U and 90Y/90Sr) and compared these fluxes to coincident benthic chamber estimates of respiration and total phosphorus efflux by quagga mussels on the lakebed. We found that advective onshore transport and vertical convective mixing increased POC and PP flux to the nearshore benthos by a factor of ~15 and ~30 over offshore trap-derived estimates of flux. From these results, we hypothesize that high benthos population densities are related to an edge effect created when the dominant mechanism of particle delivery transitions from gravitational settling to convection

Seiche- and storm-driven benthic oxygen uptake in a eutrophic freshwater bay determined with aquatic eddy covariance

Oxygen depletion in bottom waters of lakes and coastal regions is expanding worldwide. To examine the causes of hypoxia, we quantified the drivers of benthic oxygen uptake in Green Bay, Lake Michigan, USA, using 2 techniques, aquatic eddy covariance and sediment core incubation. We investigated benthic oxygen uptake along a gradient in C deposition, including shallow water near the riverine source of eutrophication and deeper waters of lower Green Bay where high net sediment deposition occurs. Time-averaged eddy covariance oxygen uptake was high near the source of eutrophication (11.5 mmol m−2 d−1) and at the shallower of the high deposition sites (9.8 mmol m−2 d−1). The eddy covariance technique revealed a decrease in benthic oxygen uptake with depth at the high deposition sites. These patterns were consistent with benthic uptake being driven by the deposition of autochthonous production. Additionally, eddy covariance revealed a nearly proportional relationship between benthic oxygen uptake and current velocity at all sites. Specifically, because of the lake seiche, water velocity typically varied 3× at a site and caused a 3× variation in benthic oxygen uptake. A summer storm also doubled bottom-water velocities and caused a further doubling of uptake to 28 mmol m−2 d−1. This high sensitivity of benthic oxygen uptake to seiche-driven water velocities indicates that redox conditions in surficial cohesive sediments are highly dynamic

Major and trace element geochemistry in Zeekoevlei, South Africa: a lacustrine record of present and past processes

This study reports a multi-parameter geochemical investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives a greater fraction of dissolved major and trace elements from natural sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sewage effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pb and Zn in this lake. The overall low dissolved metal load results from negligible industrial pollution, high pH and elevated metal uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-induced sediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/N ratio (10) indicates primarily an algal source for the sedimentary organic matter. Variation in sedimentary organic C content with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewater treatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals in sediments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased element concentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareous sediments also play an important role in adsorbing metals in Zeekoevlei sediments

How are coastal benthos fed?

Water movement can influence the distribution of benthos, in part, by increasing food delivery; however, the impact of advective transport and turbulent diffusion on organic matter flux to nearshore benthic communities is not well quantified. In this study, we measured the vertical particulate organic carbon (POC) and particulate phosphorus (PP) flux in nearshore Lake Michigan using two naturally occurring daughter/parent radionuclide pairs (234Th/238U and 90Y/90Sr) and compared these fluxes to coincident benthic chamber estimates of respiration and total phosphorus efflux by quagga mussels on the lakebed. We found that advective onshore transport and vertical convective mixing increased POC and PP flux to the nearshore benthos by a factor of ~15 and ~30 over offshore trap-derived estimates of flux. From these results, we hypothesize that high benthos population densities are related to an edge effect created when the dominant mechanism of particle delivery transitions from gravitational settling to convection

Elemental (C, N, H and P) and stable isotope (δ¹⁵N and δ¹³C) signatures in sediments from Zeekoevlei, South Africa: a record of human intervention in the lake

We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ13C and δ15N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ13Corganic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ15Norganic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ15N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l−1) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core

Data from: How are coastal benthos fed?

Water movement can influence the distribution of benthos, in part, by increasing food delivery; however, the impact of advective transport and turbulent diffusion on organic matter flux to nearshore benthic communities is not well quantified. In this study, we measured the vertical particulate organic carbon (POC) and particulate phosphorus (PP) flux in nearshore Lake Michigan using two naturally occurring daughter/parent radionuclide pairs (234Th/238U and 90Y/90Sr) and compared these fluxes to coincident benthic chamber estimates of respiration and total phosphorus efflux by quagga mussels on the lakebed. We found that advective onshore transport and vertical convective mixing increased POC and PP flux to the nearshore benthos by a factor of ∼ 15 and ∼ 30 over offshore trap-derived estimates of flux. From these results, we hypothesize that high benthos population densities are related to an edge effect created when the dominant mechanism of particle delivery transitions from gravitational settling to convection

The Influence of Sublacustrine Hydrothermal Vent Fluids on the Geochemistry of Yellowstone Lake

The geochemical composition of Yellowstone Lake water is strongly influenced by sublacustrine hydrothermal vent activity. The evidence for this conclusion is twofold. First, mass-balance calculations indicate that the outflow from Yellowstone Lake is enriched in dissolved As, B, Cl, Cs, Ge, Li, Mo, Sb, and W relative to inflowing waters. Calculations involving stable isotopes of hydrogen and oxygen (δD and δ18O, respectively) and mass-balances indicate about 13 percent evapoconcentration in the lake, which is inadequate to account for the enrichment of these elements in the water column. Second, linear relationships between the concentration of Cl and many other elements in the lake and in hydrothermal vent fluids suggest that Yellowstone Lake water is a mixture of inflowing surface water and hydrothermal source fluid. The conservative behavior of many elements is further demonstrated in mixing experiments that utilize subaerial geyser fluids and Yellowstone River water sampled at the lake outlet. The hydrothermal source fluid feeding the lake is identified by comparing theoretical predictions of the Cl and δD content of boiled, deep, thermal-reservoir fluid with observed compositions of water-column, pore-water, and vent samples from Yellowstone Lake. This comparison indicates that the hydrothermal source fluid has a temperature of 220°C and a Cl content of 570 mg/kg (~16 mM or millimoles per liter) and it evolved by boiling of a deep reservoir fluid with δD equal to –149 per mil and Cl content of 310 mg/kg. The concentrations of other elements in the hydrothermal source fluid are estimated using the observed linear relationships between Cl and other elements in lake and hydrothermal vent fluids. These concentrations indicate strong enrichment of Cl, Si, B, Li, Na, K, Rb, As, Ge, Mo, Sb, and W in sublacustrine hydrothermal vent fluids. In general, the composition of the hydrothermal source fluid is similar to the composition of subaerial geyser water in Yellowstone National Park (the Park). The Cl concentration in the hydrothermal source fluid indicates that Yellowstone Lake water is about 1 percent hydrothermal source fluid and 99 percent inflowing stream water. The flux of hydrothermal source fluid into the lake is about 8 x 109 kg of water per year, based on mass-balance calculations for Cl. If the concentration of Cl in deep reservoir fluid, rather than in hydrothermal source fluid, is used, then the flow is calculated to be 1.5x1010 kg of water per year. Using the latter estimate, sublacustrine vents in Yellowstone Lake account for ~10 percent of the total flux of deep, thermal reservoir water in the Park, as estimated from Cl in streams (Friedman and Norton, 2000, this volume). Although the volumetric input of water into the lake from hydrothermal vents is small, the impact of the vent fluids on the geochemistry of Yellowstone Lake is large because of the great enrichment of many elements in these fluids. Because about 41 million kg per day of element-enriched deep thermal water flows into the lake, and recent swath sonar studies show the presence of numerous newly recognized hydrothermal features, Yellowstone Lake should be considered one of the most significant hydrothermal basins in the Park