Abstract. Estimates of dissolved organic carbon (DOC) release by marine macrophyte communities (seagrass meadows and macroalgal beds) were obtained experimentally using in situ benthic chambers. The effect of light availability on DOC release by macrophyte communities was examined in two communities both by comparing net DOC release under light and dark, and by examining the response of net DOC release to longer-term (days) experimental shading of the communities. All most 85% of the seagrass communities and almost all of macroalgal communities examined acted as net sources of DOC. There was a weak tendency for higher DOC fluxes under light than under dark conditions in seagrass meadow. There is no relationship between net DOC fluxes and gross primary production (GPP) and net community production (NCP), however, this relationship is positive between net DOC fluxes and community respiration. Net DOC fluxes were not affected by shading of a T. testudinum community in Florida for 5 days, however, shading of a mixed seagrass meadow in the Philippines led to a significant reduction on the net DOC release when shading was maintained for 6 days compared to only 2 days of shading. Based on published and unpublished results we also estimate the global net DOC production by marine macrophytes. The estimated global net DOC flux, and hence export, from marine macrophyte is about 0.197 ± 0.015 Pg C yr−1 or 0.212 ± 0.016 Pg C yr−1 depending if net DOC flux by seagrass meadows was estimated by taking into account the low or high global seagrass area, respectively.
Stable carbon isotope measurements (δ 13 C) were used to assess the sources of carbon assimilated by the fan mussel Pinna nobilis , in sea grass Posidonia oceanica meadows, and an associated shrimp Pontonia pinnophylax which occurs within this bivalve's mantle cavity. The primary carbon sources available to both animals displayed a wide range of δ 13 C values, from −12·3 to −22·3‰. The δ 13 C and δ 15 N of Pinna nobilis and Pontonia pinnophylax suggest that they assimilate carbon from similar sources, occupy comparable trophic levels and that their association is commensal.
Along the west coast of South America, from the tropical zone to the Patagonian waters, there is a significant latitudinal gradient in seawater temperature, salinity and carbonate chemistry. These physical–chemical changes in seawater induce morphological and physiological responses in calcifying organisms, which may alter their energy budget and calcification processes. In this study, we study the organism energy maintenance (i.e. metabolic rate) and mineralogical composition of the shell of the juvenile marine snails Concholepas concholepas (Gastropoda: Muricidae), collected from benthic populations located ~2000km apart, varies across geographic regions along the Chilean coast. We found that in juvenile snails, the calcite:aragonite ratio in the pallial shell margin (i.e. newly deposited shell) increase significantly from northern to southern populations and this increase in calcite precipitation in the shell of juveniles snails was associated with a decrease in oxygen consumption rates in these populations. Our result suggests that calcite secretion may be favoured when metabolic rates are lowered, as this carbonate mineral phase might be less energetically costly for the organism to precipitate. This result is discussed in relation to the natural process such as coastal upwelling and freshwater inputs that promote geographic variation in levels of pH and carbonate saturation state in seawater along the Chilean coast.
Abstract We describe shoot density, aboveground and belowground biomass, form and leaf production per shoot over a one-year cycle for Cymodocea nodosa meadows in the Ria Formosa lagoon (South Portugal). Habitat conditions in Ria Formosa allow the development of lush meadows (up to 1752 shoots m -2 ), supporting high shoot biomass (up to 945 g DW m -2 ), high leaf productivity (up to 14 g DW m -2 d -1 ) and large shoots (up to 113 cm leaf length) despite being located at the northern limit of the distributional range in the Atlantic Ocean. Biomass in the meadows examined is higher than other Atlantic sites with C. nodosa, and comparable to the most productive seagrass meadows yet reported. Shoot morphometry and biomass distribution were variable within Ria Formosa, with the stands growing in muddy sediments having a lower ratio of belowground/aboveground biomass than those in sandy sites. C. nodosa had a clear unimodal growth cycle, reaching maximum leaf development in summer. Mean leaf length and number of leaves per shoot, the rate of appearance of new leaves and the rate of leaf fall, average leaf growth and leaf turnover rates (P/B) were minimal in February and March, and maximal in July and September. The high production of the meadows examined may be related to the high nutrient availability in Ria Formosa, evidenced by the high leaf nutrient content (3.4% of DW and 0.38% of DW for nitrogen and phosphorus, respectively).
A set of eight large (20 m3) mesocosms were moored in Johnson's Dock (62°39.576'S, 60°22.408'W, Livingston Island, Antarctica) to experimentally generate a gradient of phytoplankton biomass and production in order to test the extent of coupling between bacteria (heterotrophic Bacteria and Archaea) and phytoplankton, as well as the role of bacterial losses to protist grazers. This was achieved by imposing four light levels (100%, 50%, 25%, and 10%) in the presence or absence of nutrient additions (0.1 mol NH4Cl, 0.1 mol F6Na2Si, and 0.01 mol KH2PO4 per day per mesocosm). The experimental treatments resulted in a broad range of chlorophyll a (Chl a) (0.31–93.5 µg Chl a L-1) and average primary production rates, while bacteria responded in a much narrower range of biomass (3–447 µg C L-1) and production (0.21– 15.71 µg C L-1 d-1). Results confirm that bacteria-chlorophyll and bacterial production-primary production relationships in the Southern Ocean differ from the typical relationships applicable to aquatic ecosystems elsewhere. Bacteria respond to phytoplankton blooms, but they respond so weakly that bacterial production represents a small percentage of primary production (1–10%). Although other mechanisms might also contribute to the weak bacterial response to phytoplankton blooms, we demonstrate that the reason for it is likely the tight control of bacterial populations by their predators. Protist grazers are able to sustain faster growth rates in the cold waters of the Southern Ocean than are bacteria, thereby preventing bacteria from responding to phytoplankton blooms more forcibly.
