Summertime plankton ecology in Fram Strait - a compilation of long- and short-term observations

Between Greenland and Spitsbergen, Fram Strait is a region where cold ice-covered Polar Water exits the Arctic Ocean with the East Greenland Current (EGC) and warm Atlantic Water enters the Arctic Ocean with the West Spitsbergen Current (WSC). In this compilation, we present two different data sets from plankton ecological observations in Fram Strait: (1) long-term measurements of satellite-derived (1998–2012) and in situ chlorophyll a (chl a) measurements (mainly summer cruises, 1991–2012) plus protist compositions (a station in WSC, eight summer cruises, 1998–2011); and (2) short-term measurements of a multidisciplinary approach that includes traditional plankton investigations, remote sensing, zooplankton, microbiological and molecular studies, and biogeochemical analyses carried out during two expeditions in June/July in the years 2010 and 2011. Both summer satellite-derived and in situ chl a concentrations showed slight trends towards higher values in the WSC since 1998 and 1991, respectively. In contrast, no trends were visible in the EGC. The protist composition in the WSC showed differences for the summer months: a dominance of diatoms was replaced by a dominance of Phaeocystis pouchetii and other small pico- and nanoplankton species. The observed differences in eastern Fram Strait were partially due to a warm anomaly in the WSC. Although changes associated with warmer water temperatures were observed, further long-term investigations are needed to distinguish between natural variability and climate change in Fram Strait. Results of two summer studies in 2010 and 2011 revealed the variability in plankton ecology in Fram Strait

Seawater carbonate chemistry and microbial polysaccharide degradation during experiments with phytoplankton Emiliania huxleyi (strain PML B92/11) and natural bacteria community, 2010

With the accumulation of anthropogenic carbon dioxide (CO2), a proceeding decline in seawater pH has been induced that is referred to as ocean acidification. The ocean's capacity for CO2 storage is strongly affected by biological processes, whose feedback potential is difficult to evaluate. The main source of CO2 in the ocean is the decomposition and subsequent respiration of organic molecules by heterotrophic bacteria. However, very little is known about potential effects of ocean acidification on bacterial degradation activity. This study reveals that the degradation of polysaccharides, a major component of marine organic matter, by bacterial extracellular enzymes was significantly accelerated during experimental simulation of ocean acidification. Results were obtained from pH perturbation experiments, where rates of extracellular alpha- and beta-glucosidase were measured and the loss of neutral and acidic sugars from phytoplankton-derived polysaccharides was determined. Our study suggests that a faster bacterial turnover of polysaccharides at lowered ocean pH has the potential to reduce carbon export and to enhance the respiratory CO2 production in the future ocean

Potential effects of ocean acidification on microbial organic matter degradation during an offshore mesocosm experiment

Microbiological processes play a major role in biogeochemical cycling of organic matter in the ocean. However, little is known about the direct effects of ocean acidification on heterotrophic activity and, hence organic matter cycling through the microbial loop. To elucidate whether future ocean acidification will directly affect microbial organic matter degradation, a study was conducted examining the impact of decreasing pH on bacterial hydrolytic enzyme activities. Different acid treatments were used to induce different pCO2 concentrations in offshore mesocosms during a series of short time experiments. The study was carried out in the Baltic Sea in July 2007 in the frame of the SOPRAN (Surface Ocean PRocesses in the ANthropocene) project. Based on kinetic measurements, maximum turn-over rates (Vmax) and half-saturation constant (Km) were calculated for glucosidases, peptidases and phosphatase. A decrease in pH led to a significant increase in Vmax of carbohydrates accompanied by a decrease of peptide degradation. Possible implications of this short time changes in heterotrophic organic matter turnover due to acidification will be discussed

Coupling of transparent exopolymer particle dynamics and microbiological processes during an ocean acidification experiment in the Baltic Sea

Transparent exopolymer particles (TEP) form from dissolved precursors and significantly contribute to the pool of particulate organic carbon in the ocean. In addition, TEP are an important structural component since they provide attachment sites for microbes on a nanometer to micrometer scale. To investigate the effect of ocean acidification on the concentration and dynamics of TEP, we conducted a series of experimental studies in the Baltic Sea in summer 2007 within the frame of the SOPRAN (Surface Ocean PRocesses in the ANthropocene) project. At this time diazotrophs were the main primary producers. Thus, the relation between autotrophic N2-fixation and heterotrophic activity (uptake of radiolabeld Leucine) was determined and compared with TEP concentration measurements to elucidate how production and fate of TEP may be altered due to short term responses to acidification.We observed that the amount of TEP as well as microbiological activities were sensitive to changes in pCO2. Our results indicate a decrease of TEP concentration with increasing pCO2 under net heterotrophic conditions. Furthermore, significant correlations between TEP concentration and bacterial abundance suggest a tight coupling between the dynamics of acidic carbohydrates and bacteria dynamics.Our results imply that ocean acidification could potentially affect microbial carbon turn-over and, hence, organic matter cycling in the ocean

Microbial activities, dynamics and diversity in a changing Arctic Ocean (Fram Strait)

As climate change is expected to be extremely intense in the Arctic Ocean there is an utmost need to study food-web interactions to contribute to a better understanding of the direction and strength of biogeochemical and microbiological feedback processes. Climate change induced alterations will directly affect food-web structures and ecosystem functioning. Recent studies indicate that environmental changes like increasing temperatures as well as freshening of surface waters promote a shift in the phytoplankton community towards a dominance of smaller cells, especially of eukaryotic picoplankton. The response of oceanic ecosystems and marine carbon cycling to these changes is particularly determined by microbial loop activity. Heterotrophic bacteria, as part of the microbial loop and a crucial component of marine food webs, have a key role in controlling carbon fluxes in the oceans. Microbial activities, dynamics and diversity were studied in the area of the deep-sea long-term observatory HAUSGARTEN of the Alfred-Wegener-Institute (Fram Strait) in July 2009. The investigation area is located within a transition zone between the northern North Atlantic and the central Arctic Ocean, which separates the warm and cold water masses originating from the West Spitzbergen and the East Greenland currents. While bacterial abundance and chlorophyll a were tightly coupled, differences of the planktonic and bacterial community structures are most likely due to the heterogeneous hydrography. Warmer water masses comprise a higher genetic diversity of picoplankton, as it is also expected for bacteria. A shift towards a dominance of smaller plankton species can potentially affect the quality of organic matter and subsequently microbial cycling. Here we present data on bacterial abundance, biomass and protein production, hydrolytical enzyme activities and community structure within different size classes with respect to changing biotic and abiotic conditions in the Fram Strait

Plankton Ecology and Biogeochemistry in a Changing Arctic Ocean (PEBCAO), first results from the AWI HAUSGARTEN area (Fram Strait)

Studies of phytoplankton ecology and biogeochemical parameters have been carried out with the ice breaking vessel RV Polarstern since the nineties at various locations in the central Artcic Ocean, the Greenland Sea and the Fram Strait, however, plankton abundance and composition were determined sporadically, and only few biogeochemical components were analysed. Since rapid environmental changes due to increasing temperatures, sea ice loss and ocean acidification in the Arctic Ocean are expected, a more comprehensive impression of the impact of the anticipated changes on pelagic biological processes and the consequences for organic matter cycling is desirable. To get more detailed investigations on the pelagic system the new research group PEBCAO was created. The aim of this group is to complement the measurements of bulk variables and samples on phyto- and protozooplankton abundances by a molecular assessment of the phytoplankton diversity, including the pico- and nanoplankton allowing to better quantifying the intrusion of invading species into the polar habitat. The point measurements during cruises will serve as ground-truthing data to create basin wide satellite images focussing on the quantitative estimation of various phytoplankton functional types, which can serve as an input for modelling approaches. Furthermore, investigations on changes in the composition of organic matter (OM) including molecular analysis of OM are carried out and together with abundance and activity of key species in zooplankton will improve the export estimates under climate change. One local focus of this group is the deep-sea long-term observatory HAUSGARTEN of AWI in the Fram Strait off Svalbard, where investigations on plankton ecology and particle flux have been carried out since the 1990. These observations can be used to identify how current observed changes are related in a historical context. Here we present first results of the multidisciplinary approach form the long term observations and the studies carried out during two Polarstern cruises (ARK 24_1&2 and ARK 25_1&2) in the summer of 2009 & 2010, respectively