Abundance, biomass, production, grazing, and biometric measurements of prokaryotes, nanoflagellates, ciliates, and dinoflagellates in three areas close to the Antarctic Peninsula in spring and summer 1995/96

Two cruises were carried out during the Austral spring-summer (November 1995 - January 1996: FRUELA 95, and January - February 1996: FRUELA 96), sampling in Bellingshausen Sea, western Bransfield Strait and Gerlache Strait. We investigated whether there were any spatial (among locations) or temporal (between cruises) differences in abundance and biomass of microbial heterotrophic and autotrophic assemblages. Changes in the concentration of chlorophyll a, prokaryotes, heterotrophic and phototrophic nanoflagellates abundance and biomass were followed in the above mentioned locations close to the Antarctic Peninsula. Parallel to these measurements we selected seven stations to determine grazing rates on prokaryotes by protists at a depth coincident with the depth of maximum chlorophyll a concentration. Measuring the disappearance of fluorescent minicells over 48 h assessed grazing by the protist community. From prokaryotes grazing rates, we estimated how much prokaryotic carbon was channeled to higher trophic levels (protists), and whether this prokaryotic carbon could maintain protists biomass and growth rates. In general higher values were reported for Gerlache Strait than for the other two areas. Differences between cruises were more evident for the oligotrophic areas in Bellingshausen Sea and Bransfield Strait than in Gerlache Strait (eutrophic area). Higher values for phototrophic (at least for chlorophyll a concentration) and abundance of all heterotrophic microbial populations were recorded in Bellingshausen Sea and Bransfield Strait during late spring - early summer (FRUELA 95) than in mid-summer (FRUELA 96). However, similar results for these variables were observed in Gerlache Strait as in spring-early summer as well as in mid-summer. Also, we found differences in grazing rates on prokaryotes among stations located in the three areas and between cruises. Thus, during late spring-early summer (FRUELA 95), the prokaryotic biomass consumed from the standing stock was higher in Bellingshausen Sea (26%/day) and Gerlache Strait (18-26%/day) than in Bransfield Strait (0.68-14%/day). During mid-summer (FRUELA 96) a different pattern was observed. The station located in Bellingshausen Sea showed higher values of prokaryotic biomass consumed (11%/day) than the one located in Gerlache Strait (2.3%/day). Assuming HNF as the main prokaryotic consumers, we estimated that the prokaryotic carbon consumed by heterotrophic nanoflagellates (HNF) barely covers their carbon requirements for growth. These results suggest that in Antarctic waters, HNF should feed in other carbon sources than prokaryotes.

Classification of coral reef types in Torres Strait (GIS files in zip-archive 319 kB)

Coral reefs represent major accumulations of calcium carbonate (CaCO3). The particularly labyrinthine network of reefs in Torres Strait, north of the Great Barrier Reef (GBR), has been examined in order to estimate their gross CaCO3 productivity. The approach involved a two-step procedure, first characterising and classifying the morphology of reefs based on a classification scheme widely employed on the GBR and then estimating gross CaCO3 productivity rates across the region using a regional census-based approach. This was undertaken by independently verifying published rates of coral reef community gross production for use in Torres Strait, based on site-specific ecological and morphological data. A total of 606 reef platforms were mapped and classified using classification trees. Despite the complexity of the maze of reefs in Torres Strait, there are broad morphological similarities with reefs in the GBR. The spatial distribution and dimensions of reef types across both regions are underpinned by similar geological processes, sea-level history in the Holocene and exposure to the same wind/wave energetic regime, resulting in comparable geomorphic zonation. However, the presence of strong tidal currents flowing through Torres Strait and the relatively shallow and narrow dimensions of the shelf exert a control on local morphology and spatial distribution of the reef platforms. A total amount of 8.7 million tonnes of CaCO3 per year, at an average rate of 3.7 kg CaCO3 m-2 yr-1 (G), were estimated for the studied area. Extrapolated production rates based on detailed and regional census-based approaches for geomorphic zones across Torres Strait were comparable to those reported elsewhere, particularly values for the GBR based on alkalinity-reduction methods. However, differences in mapping methodologies and the impact of reduced calcification due to global trends in coral reef ecological decline and changing oceanic physical conditions warrant further research. The novel method proposed in this study to characterise the geomorphology of reef types based on classification trees provides an objective and repeatable data-driven approach that combined with regional census-based approaches has the potential to be adapted and transferred to different coral reef regions, depicting a more accurate picture of interactions between reef ecology and geomorphology.