Leon Moodley

Subsurface activity of benthic foraminifera in relation to porewater oxygen content: laboratory experiments

In a series of laboratory experiments, we examined the migratory activity of some common benthic foraminifera in relation to subsurface oxygen concentrations in porewaters. Our results clearly demonstrate that foraminifera are capable of migrating through anoxic sediments, reinforcing the idea that some common benthic foraminifera are facultative anaerobes. We observed that migration was not completely random; there seemed to be a bias towards upward migration when buried without a subsurface oxic zone. Although this migration can be taken as a response to the absence of oxygen, it was not directly linked to gradients of oxygen as these were not present at that depth and a large portion of the assemblages continued to inhabit deeper sediment layers. The relatively higher percentage of soft-shelled foraminifera in the oxic zone suggests that they may be less tolerant to anoxic conditions than hard-shelled foraminifera. This possible differential tolerance may be one factor that will determine the survival or success of different species buried in deeper sediment layers.

Carbon flows through a benthic food web: Integrating biomass, isotope and tracer data.

The herbivorous, detrital and microbial pathways are major components of marine food webs. Although it is commonly recognized that these pathways can be linked in several ways, elucidating carbon transfers between or within these pathways remains a challenge. Intertidal flat communities are driven by a wide spectrum of organic matter sources that support these different pathways within the food web. Here we reconstruct carbon pathways using inverse analysis based on mass balancing, stable isotope signatures and tracer data. Data were available on biomass, total carbon production and processing, integrated diet information from stable isotope signatures and the transfer of recently produced carbon through the food web from an isotope tracer study. The integration of these data improved the quality of the inverse food web reconstruction considerably, as demonstrated explicitly by an uncertainty analysis. Deposition of detritus (detrital pathway) from the water column and subsequent assimilation and respiration by bacteria and to a lesser extent by microbenthos (microbial pathway) dominated the food web. Secondary production was dominated by bacteria (600 mg C m −2 d −1 ), but transfer to higher trophic levels was limited to 9% and most bacterial carbon was recycled back to dissolved organic carbon (DOC) and detritus. Microbenthos secondary production (77 mg C m −2 d −1 ) was supported by DOC (73%) and detritus (26%) and was entirely transferred up the food web. The higher trophic levels consisting of nematodes, meiobenthos (copepods, ostracods and foraminifera) and macrobenthos fed highly selectively and relied primarily on microphytobenthos and pelagic primary production (herbivorous pathway). Deposit feeding is a common feeding mode among these sediment dwelling fauna, but detritivory was negligible due to this selective feeding. This strong resource selectivity implies that the herbivorous and detrital-microbial pathways function more or less autonomously, with limited interaction.

Similar rapid response to phytodetritus deposition in shallow and deep-sea sediments

The short-term benthic response to an input of fresh organic matter was examined in vastly contrasting benthic environments (estuarine intertidal to deep-sea) using 13C-labeled diatoms as a tracer of labile carbon. Benthic processing was assessed in major compartments through 13C-enrichment in CO2, in bacteria-specific phospholipids and in fauna tissue. A rapid response was evident in all environments. Under warm bottom water (14–18°C), similar quantities of the added carbon were respired within 24 hours in shallow and deep-sea sediments. However, the speed and magnitude of respiration were strongly reduced under low bottom water temperature (4–6°C), both in a shallow and a deep-sea site. Rapid carbon respiration even in deep-sea sediments almost devoid of fauna highlights the key role of bacteria, the most ubiquitous benthic component, in this short-term respiration of fresh organic matter. However, when present, fauna rapidly ingest algal material, thereby increasing the amount of carbon processed and directly extending carbon flow pathways.

Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consumption, macro- and meiofauna dynamics

Benthic activity was examined at three stations (18 m water depth) in the northwestern Adriatic Sea. Carbon mineralisation rates, as based on sediment oxygen consumption rates, ranged from 54 to 89 g C m 2 y 1 . The relatively high carbon mineralisation rates, large macrofaunal biomass (9 to 16 g C m 2 ) and macrofaunal production (11 to 19 g Cm 2 y 1 ) provide evidence of high organic-matter input and intense benthic‐pelagic coupling. This is further supported by the high dominance of the suspension-feeding bivalve Corbula gibba, which accounts for 52 to 63% of the total annual macrofaunal biomass production. Although the infaunal distribution of total macrofauna showed a sharp decline in densities and biomass with depth into the sediment, different patterns within the dominant taxa were observed. Whilst the bivalve Corbula gibba and the amphipod Ampelisca sp. were restricted to the surface layer, other species such as the dominant bivalve Mysella sp. and the gastropod Hyala sp. were not confined to a specific depth level and the majority of the populations occurred deeper than 5 cm into the sediment. Bioturbation, based on the occurrence of macrofauna, extended to at least 20 cm. Nematodes and foraminifera together formed 80 to 90% of the meiofaunal community in the upper 5 cm of the sediment. Annual mean densities ranged from 3.40 to 6:07 10 6 ind. m 2 . Maximum abundance of meiofauna was not encountered at the station where maximum macrofaunal activity was recorded, and this could reflect the negative effect of biological interaction on meiofaunal densities in areas that have a high food supply.

Oxygenation and organic-matter preservation in marine sediments: Direct experimental evidence from ancient organic carbon–rich deposits

Clarification of the factors involved in the formation of unusual ancient organic carbon- rich deposits (like eastern Mediterranean sapropels) is central in understanding oceanic carbon cycling. The role of oxygenation remains a subject of controversy primarily due to two major uncertainties: (1) it is unknown if ancient organic-rich deposits reflect an accumulation of refractory organic matter (OM) or oxygenation-related aberrant sediment OM recycling, and (2) although marine OM degradation may be slower under anoxic conditions, its ultimate impact on organic carbon (Corg) preservation over geological time remains unclear. Here we provide direct experimental evidence that the Corg in eastern Mediterranean S1 sapropels (deposited .5 ka) is still highly reactive and that a shutdown in labile organic matter degradation under anoxic conditions played a key role in the formation of these deposits.

Coral cavity sponges depend on reef-derived food resources: stable isotope and fatty acid constraints

The diet of cavity sponges on the narrow fringing reefs of Curaçao, Caribbean was studied. The origin and resources of the bulk food of these sponges, i.e., dissolved organic matter (DOM), were identified using stable carbon and nitrogen isotopes and fatty acid biomarkers. We found that phytoplankton and its derived DOM from the adjacent open sea and from reef overlying water is not the main source of food for most of the sponges examined nor is bacterioplankton. Interestingly, dual stable isotope signatures (δ13Corg, δ15Norg) and fatty acid biomarkers appoint coral mucus and organic matter derived from crustose coralline algae (CCA) as probable food sources for encrusting sponges. Mucus-derived DOM may contribute up to 66% to the diet of examined sponges based on results of dual isotope mixing model analysis. The contribution of CCA (as purported representative for benthic algae) was smaller with values up to 31%. Together, mucus- and CCA-derived substrates contributed for 48–73% to the diet of sponges. The presence of the exogenous fatty acid 20:4ω6 in sponges, which is abundant in coral mucus of Madracis mirabilis and in CCA, highlights these reef-derived resources as sources of nutrition for DOM feeding cavity sponges. The relatively high concentrations of exogenous 20:4ω6 in all sponges examined supports our hypothesis that the bulk of the food of the cavity sponge community is reef-derived. Our results imply that cavity sponges play an important role in conserving food and energy produced within the reef.

Vertical distribution of meiofauna in sediments from contrasting sites in the Adriatic Sea: Clues to the role of abiotic versus biotic control.

Abstract The vertical profiles of dominant metazoan meiobenthic taxa were studied at four contrasting sites in the Adriatic Sea in relation to macrofaunal activity and oxygen penetration depth in the sediment. Vertical profiles of copepods were related to the oxygen penetration depth and showed peak densities in the upper cm or deeper in sediments with a broader oxic zone. In contrast, nematodes penetrated much deeper into the sediment and no direct relationship with oxygen penetration depth was evident. Nematode penetration depths were related to or reflected the differences in macrofaunal activity or mixing at the four sites which appears to be an important factor governing the subsurface activity of nematodes.

Impaired short-term functioning of a benthic community from a deep Norwegian fjord following deposition of mine tailings and sediments

The extraction of minerals from land-based mines necessitates the disposal of large amounts of mine tailings. Dumping and storage of tailings into the marine environment, such as fjords, is currently being performed without knowing the potential ecological consequences. This study investigated the effect of short-term exposure to different deposition depths of inert iron ore tailings (0.1, 0.5 and 3 cm) and dead subsurface sediment (0.5 and 3 cm) on a deep water (200 m) fjord benthic assemblage in a microcosm experiment. Biotic and abiotic variables were measured to determine structural and functional changes of the benthic community following an 11 and 16 day exposure with tailings and dead sediment, respectively. Structural changes of macrofauna, meiofauna and bacteria were measured in terms of biomass, density, community composition and mortality while measures of oxygen penetration depth, sediment community oxygen consumption and 13C-uptake and processing by biota revealed changes in the functioning of the system. Burial with mine tailings and natural sediments modified the structure and functioning of the benthic community albeit in a different way. Mine tailings deposition of 0.1 cm and more resulted in a reduced capacity of the benthic community to remineralize fresh 13C-labelled algal material, as evidenced by the reduced sediment community oxygen consumption and uptake rates in all biological compartments. At 3 cm of tailings deposition, it was evident that nematode mortality was higher inside the tailings layer, likely caused by reduced food availability. In contrast, dead sediment addition led to an increase in oxygen consumption and bacterial carbon uptake comparable to control conditions, thereby leaving deeper sediment layers anoxic and in turn causing nematode mortality at 3 cm deposition. This study clearly shows that even small levels (0.1 cm) of instantaneous burial by mine tailings may significantly reduce benthic ecosystem functioning on the short term. Furthermore, it reveals the importance of substrate characteristics and origin when studying the effects of substrate addition on marine benthic fauna. Our findings should alert decision makers when considering approval of new deep-sea tailings placement sites as this technique will have major negative impacts on benthic ecosystem functioning over large areas.

Effects of chronic crude oil exposure on early developmental stages of the Northern krill (Meganyctiphanes norvegica)

ABSTRACT Rising oil and gas activities in northern high latitudes have led to an increased risk of petroleum pollution in these ecosystems. Further, seasonal high UV radiation at high latitudes may elevate photo-enhanced toxicity of petroleum pollution to marine organisms. Zooplanktons are a key ecological component of northern ecosystems; therefore, it is important to assess their sensitivity to potential pollutants of oil and gas activity. As ontogenetic development may be particularly sensitive, the aim of this study was to examine the impact of chronic exposure to oil water dispersion (OWD) on development and feeding of early life stages of the Northern krill, Meganyctiphanes norvegica. In a range of experiments, embryonic, nonfeeding, and feeding larval stages were exposed to concentrations of between 0.01 and 0.1 mg/L of oil or photo-modified oil for 19 and 21 d. No significant effects on egg respiration, hatching success, development, length and larval survival were observed from these treatments. Similarly, evolution of fatty acid composition patterns during ontogenetic development was unaffected. The results indicates a high degree of resilience of these early developmental stages to such types and concentrations of pollutants. However, feeding and motility in later calyptopis-stage larvae were significantly impaired at exposure of 0.1 mg/L oil. Data indicate that feeding larval stage of krill was more sensitive to OWD than early nonfeeding life stages. This might be attributed to the narcotic effects of oil pollutants, their direct ingestion, or accumulated adverse effects over early development.