Community structure and biodiversity in marine benthic communities during experiments, 2011

A mesocosm experiment was conducted to quantify the effects of reduced pH and elevated temperature on an intact marine invertebrate community. Standardised faunal communities, collected from the extreme low intertidal zone using artificial substrate units, were exposed to one of eight nominal treatments (four pH levels: 8.0, 7.7, 7.3 and 6.7, crossed with two temperature levels: 12 and 16°C). After 60 days exposure communities showed significant changes in structure and lower diversity in response to reduced pH. The response to temperature was more complex. At higher pH levels (8.0 and 7.7) elevated temperature treatments contained higher species abundances and diversity than the lower temperature treatments. In contrast, at lower pH levels (7.3 and 6.7), elevated temperature treatments had lower species abundances and diversity than lower temperature treatments. The species losses responsible for these changes in community structure and diversity were not randomly distributed across the different phyla examined. Molluscs showed the greatest reduction in abundance and diversity in response to low pH and elevated temperature, whilst annelid abundance and diversity was mostly unaffected by low pH and was higher at the elevated temperature. The arthropod response was between these two extremes with moderately reduced abundance and diversity at low pH and elevated temperature. Nematode abundance increased in response to low pH and elevated temperature, probably due to the reduction of ecological constraints, such as predation and competition, caused by a decrease in macrofaunal abundance. This community-based mesocosm study supports previous suggestions, based on observations of direct physiological impacts, that ocean acidification induced changes in marine biodiversity will be driven by differential vulnerability within and between different taxonomical groups. This study also illustrates the importance of considering indirect effects that occur within multispecies assemblages when attempting to predict the consequences of ocean acidification and global warming on marine communities.

Quantification and molecular characterization of dissolved organic sulfur in water samples obtained during POLARSTERN cruise ANT-XXV (PS73) to the East Atlantic and the Southern Ocean

Although sulfur is an essential element for marine primary production and critical for climate processes, little is known about the oceanic pool of non-volatile dissolved organic sulfur (DOS). We present a basin-scale distribution of solid phase extractable DOS in the East Atlantic Ocean and the Atlantic sector of the Southern Ocean. While molar DOS versus dissolved organic nitrogen (DON) ratios of 0.11 ± 0.024 in Atlantic surface water resembled phytoplankton stoichiometry (S/N ~ 0.08), increasing dissolved organic carbon (DOC) versus DOS ratios and decreasing methionine-S yield demonstrated selective DOS removal and active involvement in marine biogeochemical cycles. Based on stoichiometric estimates, the minimum global inventory of marine DOS is 6.7 Pg S, exceeding all other marine organic sulfur reservoirs by an order of magnitude.

Coccolithophore sensitivities to changing carbonate chemistry - an ecological framework

Coccolithophores are a group of unicellular phytoplankton species whose ability to calcify has a profound influence on biogeochemical element cycling. Calcification rates are controlled by a large variety of biotic and abiotic factors. Among these factors, carbonate chemistry has gained considerable attention during the last years as coccolithophores have been identified to be particularly sensitive to ocean acidification. Despite intense research in this area, a general concept harmonizing the numerous and sometimes (seemingly) contradictory responses of coccolithophores to changing carbonate chemistry is still lacking to date. Here, we present the "substrate-inhibitor concept" which describes the dependence of calcification rates on carbonate chemistry speciation. It is based on observations that calcification rate scales positively with bicarbonate (HCO3-), the primary substrate for calcification, and carbon dioxide (CO2), which can limit cell growth, whereas it is inhibited by protons (H+). This concept was implemented in a model equation, tested against experimental data, and then applied to understand and reconcile the diverging responses of coccolithophorid calcification rates to ocean acidification obtained in culture experiments. Furthermore, we (i) discuss how other important calcification-influencing factors (e.g. temperature and light) could be implemented in our concept and (ii) embed it in Hutchinson's niche theory, thereby providing a framework for how carbonate chemistry-induced changes in calcification rates could be linked with changing coccolithophore abundance in the oceans. Our results suggest that the projected increase of H+ in the near future (next couple of thousand years), paralleled by only a minor increase of inorganic carbon substrate, could impede calcification rates if coccolithophores are unable to fully adapt. However, if calcium carbonate (CaCO3) sediment dissolution and terrestrial weathering begin to increase the oceans' HCO3- and decrease its H+ concentrations in the far future (10 -100 kyears), coccolithophores could find themselves in carbonate chemistry conditions which may be more favorable for calcification than they were before the Anthropocene.

Seawater carbonate chemistry and growth, calcification of Thoracosphaera heimii in a laboratory experiment

Ocean acidification is considered a major threat to marine ecosystems and may particularly affect calcifying organisms such as corals, foraminifera and coccolithophores. Here we investigate the impact of elevated pCO2 and lowered pH on growth and calcification in the common calcareous dinoflagellate Thoracosphaera heimii. We observe a substantial reduction in growth rate, calcification and cyst stability of T. heimii under elevated pCO2. Furthermore, transcriptomic analyses reveal CO2 sensitive regulation of many genes, particularly those being associated to inorganic carbon acquisition and calcification. Stable carbon isotope fractionation for organic carbon production increased with increasing pCO2 whereas it decreased for calcification, which suggests interdependence between both processes. We also found a strong effect of pCO2 on the stable oxygen isotopic composition of calcite, in line with earlier observations concerning another T. heimii strain. The observed changes in stable oxygen and carbon isotope composition of T. heimii cysts may provide an ideal tool for reconstructing past seawater carbonate chemistry, and ultimately past pCO2. Although the function of calcification in T. heimii remains unresolved, this trait likely plays an important role in the ecological and evolutionary success of this species. Acting on calcification as well as growth, ocean acidification may therefore impose a great threat for T. heimii.

Fork lengths, stomach contents and parasites of ninespine stickleback and arctic char in Iqalugaajuruluit Lake, Canada

Stable d13C and d15N isotopes, diet and parasites demonstrated that the prey consumed by ninespine stickleback Pungitius pungitius in a small lake on Baffin Island changed during the summer and also revealed intraspecific variation in their ecological niche. In July, there were differences in the diets of male and female ninespine stickleback as indicated by the stable isotopes, differences corroborated by the data on diet composition and the parasite fauna. Differences suggested that the sexes occupied different habitats during spawning. During July, females utilise the shallower littoral areas consuming zooplankton and benthic organisms, while males occupy deeper areas of the littoral zone feeding mainly on pelagic zooplankton. Parasite data support these observations as males had higher infections of copepod-transmitted parasites than females. There appeared to be no segregation of resources between males and females in late August, although the diet of both male and female ninespine stickleback shifted towards more benthic organisms, compared with July. Differences in d13C isotope, diet composition and infections of co-occurring parasites demonstrated that sympatric ninespine stickleback and Arctic char Salvelinus alpinus captured in the littoral zone occupied separate niches. Ninespine stickleback preyed mainly on zooplankton and chironomids, while Arctic char consumed a greater variety of prey items, including zooplankton and larger-sized prey such as insects and ninespine stickleback. The multifaceted approach improved our understanding of the trophic ecology of ninespine stickleback in southern Baffin Island and quantified resource use and dietary overlap with Arctic char.