Kay Vopel

Deposit-feeding sea cucumbers enhance mineralization and nutrient cycling in organically-enriched coastal sediments.

Background Bioturbators affect multiple biogeochemical interactions and have been suggested as suitable candidates to mitigate organic matter loading in marine sediments. However, predicting the effects of bioturbators at an ecosystem level can be difficult due to their complex positive and negative interactions with the microbial community. Methodology/Principal Findings We quantified the effects of deposit-feeding sea cucumbers on benthic algal biomass (microphytobenthos, MPB), bacterial abundance, and the sediment–seawater exchange of dissolved oxygen and nutrients. The sea cucumbers increased the efflux of inorganic nitrogen (ammonium, NH4 +) from organically enriched sediments, which stimulated algal productivity. Grazing by the sea cucumbers on MPB (evidenced by pheopigments), however, caused a net negative effect on primary producer biomass and total oxygen production. Further, there was an increased abundance of bacteria in sediment with sea cucumbers, suggesting facilitation. The sea cucumbers increased the ratio of oxygen consumption to production in surface sediment by shifting the microbial balance from producers to decomposers. This shift explains the increased efflux of inorganic nitrogen and concordant reduction in organic matter content in sediment with bioturbators. Conclusions/Significance Our study demonstrates the functional role and potential of sea cucumbers to ameliorate some of the adverse effects of organic matter enrichment in coastal ecosystems.

Interactions between the mat-forming alga Didymosphenia geminata and its hydrodynamic environment

Benthic autotrophs in oligotrophic rivers must adapt to and modify their hydrodynamic environment to balance the conflicting requirements of minimal drag (to minimize detachment risks) and maximal exposure to turbulent flow (to maximize nutrient acquisition). We explored flow–organism interactions using the benthic, freshwater alga Didymosphenia geminata. D. geminata forms large mats in swift, oligotrophic alluvial rivers. The physical properties that allow D. geminata to resist detachment and proliferate under these harsh conditions are unknown. We transplanted cobbles with attached D. geminata mats from a riverbed to a flume and used velocimetry and microelectrode profiling to measure hydrodynamic and transport conditions above and within the mats over a wide range of flows. We then removed the mats from the cobbles and repeated the velocimetry measurements. Experiment results indicated that D. geminata mats reduce form-induced stresses and near-bed turbulent velocity fluctuations, which may reduce the risk of detachment. D. geminata mats also increase turbulent shear stress just above mat surfaces, which may enhance water column–mat solute exchange. High friction associated with flow at mat surfaces leads to very low velocities and predominantly diffusive transport within mats, which may in turn favor the retention of solutes derived from organic matter within and below mats. Enhanced mass transfer at mat surfaces and effective solute retention in mat matrices suggest a mechanism by which D. geminata cells acquire nutrients from different sources: advection-dominated transport of water-column nutrients to cells at mat surfaces, and diffusion-dominated transport from decomposing organic matter within mats, with minimal advective losses.

The large-scale environmental impact experiment DISCOL—reflection and foresight

It is now accepted that environmental impact studies should accompany societys growing interest in exploiting deep-sea resources. A large-scale experiment, DISCOL (Disturbance and recolonisation experiment in a manganese nodule area of the deep South Pacific) was conducted to evaluate potential impacts from mining on the deep-sea bed. DISCOL was the first of a series of projects aimed at better understanding impacts of industrial-scale mining of polymetallic nodules upon the seafloor and its biological community. A schedule of biological work, including a disturbance scheme and sampling patterns, for another 12-year period is described that builds on the DISCOL results, but is strictly valid only for this area. However, future experiments may use estimates from the DISCOL data as a first approximation in their planning phase, but will need to conduct site-specific sampling to establish a baseline.

Sediment–seawater solute flux in a polluted New Zealand estuary

We investigated the sediment-seawater solute flux at five sites in the polluted Avon-Heathcote Estuary, New Zealand, to provide a point of comparison for future studies of the effects of the closure of a major wastewater outfall into the estuary. Sediments collected in winters 2007 and 2008, and summer 2008, ranked consistently in organic matter content. Microelectrode profiling and sediment-core incubations revealed (1) a dominant role of microphytes in regulating solute flux causing significant differences in the dark and light sediment O₂ consumption (R(d), R(l)), total sediment O₂ utilisation (TOU(d), TOU(l)), and inorganic nutrient flux, (2) consistent ranking of sites in solute flux, and (3) a clear solute-flux signature of the wastewater effluent. Sediment near the wastewater outfall exhibited the highest absolute R and TOU(,) the lowest ratio R(l)/R(d,) the highest dark efflux of dissolved reactive phosphorus and ammonium, and the highest dark and light uptake of nitrate+nitrite.

Sulfide assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum

We investigated the constraints on sulfide uptake by bacterial ectosymbionts on the marine peritrich ciliate Zoothamnium niveum by a combination of experimental and numerical methods. Protists with symbionts were collected on large blocks of mangrove-peat. The blocks were placed in a flow cell with flow adjusted to in situ velocity. The water motion around the colonies was then characterized by particle tracking velocimetry. This shows that the feather-shaped colony of Z. niveum generates a unidirectional flow of seawater through the colony with no recirculation. The source of the feeding current was the free-flowing water although the size of the colonies suggests that they live partly submerged in the diffusive boundary layer. We showed that the filtered volume allows Z. niveum to assimilate sufficient sulfide to sustain the symbiosis at a few micromoles per liter in ambient concentration. Numerical modeling shows that sulfide oxidizing bacteria on the surfaces of Z. niveum can sustain 100-times higher sulfide uptake than bacteria on flat surfaces, such as microbial mats. The study demonstrates that the filter feeding zooids of Z. niveum are preadapted to be prime habitats for sulfide oxidizing bacteria due to Z. niveum’s habitat preference and due to the feeding current. Z. niveum is capable of exploiting low concentrations of sulfide in near norm-oxic seawater. This links its otherwise dissimilar habitats and makes it functionally similar to invertebrates with thiotrophic symbionts in filtering organs.

The Roles of Sea-Ice, Light and Sedimentation in Structuring Shallow Antarctic Benthic Communities

On polar coasts, seasonal sea-ice duration strongly influences shallow marine environments by affecting environmental conditions, such as light, sedimentation, and physical disturbance. Sea-ice dynamics are changing in response to climate, but there is limited understanding of how this might affect shallow marine environments and benthos. Here we present a unique set of physical and biological data from a single region of Antarctic coast, and use it to gain insights into factors shaping polar benthic communities. At sites encompassing a gradient of sea-ice duration, we measured temporal and spatial variation in light and sedimentation and hard-substrate communities at different depths and substrate orientations. Biological trends were highly correlated with sea-ice duration, and appear to be driven by opposing gradients in light and sedimentation. As sea-ice duration decreased, there was increased light and reduced sedimentation, and concurrent shifts in community structure from invertebrate to algal dominance. Trends were strongest on shallower, horizontal surfaces, which are most exposed to light and sedimentation. Depth and substrate orientation appear to mediate exposure of benthos to these factors, thereby tempering effects of sea-ice and increasing biological heterogeneity. However, while light and sedimentation both varied spatially with sea-ice, their dynamics differed temporally. Light was sensitive to the site-specific date of sea-ice breakout, whereas sedimentation fluctuated at a regional scale coincident with the summer phytoplankton bloom. Sea-ice duration is clearly the overarching force structuring these shallow Antarctic benthic communities, but direct effects are imposed via light and sedimentation, and mediated by habitat characteristics.

Trace metal cycling in the Whau Estuary, Auckland, New Zealand

Environmental context. The accumulation of trace metals from urban runoff is a serious environmental concern. In the present paper we show that, in the case of the Whau Estuary, Auckland, New Zealand, there is a significant particulate Zn input, of which a significant amount of Zn is lost from the particulate phase into the dissolved phase within the water column, and via molecular diffusion across the water–sediment interface. The present study shows that changes in the chemical speciation of Zn, associated with changes in salinity, play a major role in regulating the recycling of this metal between the particulate and dissolved phases. Abstract. Dissolved Zn, Cd, Cu, Fe, and Pb concentrations were measured along a salinity gradient in the Whau Estuary, Auckland, New Zealand. We found a mid-salinity maximum in dissolved Zn and Cd concentrations, consistent with significant loss of these metals from the particulate phase into the dissolved phase. Changes in the chemical speciation of these two metals were coupled to changes in salinity and this was the major driver for Zn and Cd loss from particulate material. Contrastingly, Cu concentrations were conservative with salinity, whereas there was significant scavenging of Fe and Pb from the dissolved phase into the particulate phase. Analysis of sediment pore-water metal concentrations indicated a peak in Zn concentration within the suboxic layer. The peak occurred at a shallower depth than those for Mn and Fe. The concentration gradient across the sediment–water interface suggests that diffusional loss of Zn from the sediment pore water into the overlying water column was occurring. Conversely, the diffusion of Cu from the water column into the sediment pore water was likely to occur because pore-water Cu concentrations were lower than the overlying water column concentrations. The results from the present study show the importance of chemical speciation and the lability of metals attached to particulate material as potentially being a critical determinant on sediment metal concentrations.

Assessing the Sulfide Footprint of Mussel Farms with Sediment Profile Imagery: A New Zealand Trial.

Growing numbers and increased stocking of marine mussel farms make reliable techniques for environmental effect assessment a priority. Previously, we showed how the color intensity of soft sediment could be used to estimate its acid volatile sulfide (AVS) content, a product of the anaerobic microbial degradation of organic matter deposits. We then proposed to include assessments of the AVS farm footprint in marine farm monitoring, in particular, to investigate temporal changes in the extent of the seafloor area of elevated sediment AVS content. Such assessment requires accurate detection of the AVS footprint boundary. Here, we demonstrate how to detect this boundary with analyses of sediment color intensity. We analyzed 182 sediment profile images taken along three transects leading from approximately 50 m inside to 200 m outside a long-line mussel farm in New Zealand and found that the mean sediment color intensity inside the farm boundary was almost one third lower than that of the sediment distant from the farm. Segmented regression analysis of the combined color intensity data revealed a breakpoint in the trend of increasing grey values with increasing distance from the farm at 56 ± 13 m (± 95% confidence interval of the breakpoint) outside the mussel farm. Statistical analyses indicated that the extent of the color intensity footprint was a function of water column depth, as was shown visually using mapping methods; organic particles disperse further in a deeper seawater column. We conclude that for soft coastal sediments, our sampling and data analysis techniques may provide a rapid and reliable supplement to existing benthic surveys that assess environmental effects of mussel farms.

Effects of CO 2 enrichment on benthic primary production and inorganic nitrogen fluxes in two coastal sediments

Ocean acidification may alter the cycling of nitrogen in coastal sediment and so the sediment–seawater nitrogen flux, an important driver of pelagic productivity. To investigate how this perturbation affects the fluxes of NOX− (nitrite/nitrate), NH4+ and O2, we incubated estuarine sand and subtidal silt in recirculating seawater with a CO2-adjusted pH of 8.1 and 7.9. During a 41-day incubation, the seawater kept at pH 8.1 lost 97% of its NOX− content but the seawater kept at pH 7.9 lost only 18%. Excess CO2 increased benthic photosynthesis. In the silt, this was accompanied by a reversal of the initial NOX− efflux into influx. The estuarine sand sustained its initial NOX− influx but, by the end of the incubation, released more NH4+ at pH 7.9 than at pH 8.1. We hypothesise that these effects share a common cause; excess CO2 increased the growth of benthic microalgae and so nutrient competition with ammonia oxidising bacteria (AOB). In the silt, diatoms likely outcompeted AOB for NH4+ and photosynthesis increased the dark/light fluctuations in the pore water oxygenation inhibiting nitrification and coupled nitrification/denitrification. If this is correct, then excess CO2 may lead to retention of inorganic nitrogen adding to the pressures of increasing coastal eutrophication.

Offshore iron sand extraction in New Zealand: Potential trace metal exposure of benthic and pelagic biota

Plans to exploit an offshore source of iron sand in South Taranaki Bight (STB), New Zealand, caused concerns that such exploitation may expose benthic and pelagic biota to elevated trace metal concentrations. We conducted dilute-acid extractions and standard elutriate tests to investigate the potential of this exploitation to (1) create a new seafloor with elevated trace metal content, (2) mobilise trace metals during iron sand extraction and, (3) enrich the returning process seawater, which feeds iron sand through mills, with trace metals. We found that recruits of freshly uncovered sediment may encounter higher-than-natural concentrations of cadmium, nickel and chromium (but not of copper, lead, and zinc) and propose to investigate the bioavailability of these metals. Elutriate test with raw and milled iron sand revealed that, for nickel and copper, dilution of the process seawater may be required to meet the local water quality guideline. We argue that this dilution can be achieved by adjustment of the mass and seawater balance of the offshore extraction process.