Nutrient-Replete Benthic Microalgae as a Source of Dissolved Organic Carbon to Coastal Waters

Abstract

Dissolved organic carbon (DOC) flux dynamics were examined in the context of other biogeochemical cycles in intertidal sediments inhabited by benthic microalgae. In August 2003, gross oxygenic photosynthetic (GOP) rates, oxygen penetration depths, and benthic flux rates were quantified at seven sites along the Duplin River, GA, USA. Sediments contained abundant benthic microalgal (BMA) biomass with a maximum chlorophyll a concentration of 201 mg chl a m −2 . Oxygen microelectrodes were used to determine GOP rates and O 2 penetration depth, which were tightly correlated with light intensity. Baseline and 15 N-nitrate amended benthic flux core incubations were employed to quantify benthic fluxes and to investigate the impact of BMA on sediment water exchange under nitrogen (N)-limited and N-replete conditions. Unamended sediments exhibited tight coupling between GOP and respiration and served as a sink for water column dissolved inorganic nitrogen (DIN) and a source of silicate and dissolved inorganic carbon (DIC). The BMA response to the N addition indicated sequential nutrient limitation, with N limitation followed by silicate limitation. In diel (light-dark) incubations, biological assimilation accounted for 83% to 150% of the nitrate uptake, while denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) accounted for <7%; in contrast, under dark conditions, DNF and DNRA accounted for >40% of the NO 3 − uptake. The N addition shifted the metabolic status of the sediments from a balance of autotrophy and heterotrophy to net autotrophy under diel conditions, and the sediments served as a sink for water column DIN, silicate, and DIC but became a source of DOC, suggesting that the increased BMA production was decoupled from sediment bacterial consumption of DOC.

Transformation and fate of microphytobenthos carbon in subtropical shallow subtidal sands: A 13C‐labeling study

“…MPB are a potential source of water-column DOC (Porubsky et al 2008). However, DOC derived from MPB represented only a small fraction of the total DOC efflux from sediments in the current study (0.4% during the first dark incubation, when excess 13 C flux as DOC was highest) and was a minor pathway for loss of 13 C fixed by MPB (2.64% over 33 d).…”

Combined Gel Probe and Isotope Labeling Technique for Measuring Dissimilatory Nitrate Reduction to Ammonium in Sediments at Millimeter-Level Resolution

“…Until now, the influence of chemical factors on DNRA and denitrification in sediments has been assessed by slurry incubations (4,12,30), by flux measurements with sealed sediment cores (7, 47) or flowthrough sediment cores (16,27,37), and in one case, in reconstituted sediment cores sliced at centimeter-level resolution (39). Here, we present a new method, the combined gel probe and isotope labeling technique, to determine the vertical distribution of the net rates of DNRA in sediments.…”

“…Conventional methods to investigate the relative importance of different pathways of dissimilatory NO 3 Ϫ reduction in sediments include slurry incubations (4,12,30), methods that use sealed sediment cores (7,47), and methods that use flowthrough sediment cores (16,27,37). Only one study used reconstituted sediment cores in which DNRA activity was measured at centimeter-level resolution (39).…”

In Situ Benthic Nutrient Flux and Sediment Oxygen Demand in Barnegat Bay, New Jersey

“…Diverse assemblages of benthic algae and microalgae have been reported to live at the sediment-water interface, and those assemblages can greatly alter the mass transfer of remineralized nutrients from the buried sediments to the overlying water column (McGlathery, Sundbäck, and Anderson, 2004;Porubsky, Veasquez, and Joye, 2008;Seitzinger and Pilling, 1992;Sundbäck et al, 1991;Sundbäck and Granéli, 1988;Yarbro and Carlson, 2008). Studies have reported that the biomass associated with a thin layer of benthic algae can account for nearly one-third of the primary production in estuarine systems and that benthic algae can be distributed widely across open-sand bottom habitats (Cahoon and Cooke, 1992;Pinckney and Zingmark, 1993).…”