Benthic algae control sediment—water column fluxes of organic and inorganic nitrogen compounds in a temperate lagoon

Abstract

Coastal lagoons are a common land-margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic-pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH , NO , DON, urea, and dissolved free and combined amino acids [DFAA,DCAA]) and O 2 were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high-nutrient and sandy, low-nutrient sites that were both dominated by benthic microalgae, and a mid-lagoon site with fine sands covered by dense macroalgal mats. Sediment-water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment-water column fluxes of DIN, urea, and DFAA, which accounted for 27-75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes-hours) and long (months) time scales. Microalgae and macroalgae clearly regulate benthic-pelagic coupling and thereby influence transformations and retention of N moving across the land-sea interface.

Ecosystem metabolism along a colonization gradient of eelgrass (Zostera marina) measured by eddy correlation

“…1). This differs from results of previous core incubation studies in the VCR lagoons that have shown unvegetated sediments were net autotrophic during at least part of the year (McGlathery et al 2001;Tyler et al 2003;Giordano et al 2012), but agrees well with the results found by Hume et al (2011) using the eddy correlation technique (see Fig. 2).…”

“…During June 2012, O 2 fluxes were also estimated from sediment core incubations for the bare site (Tyler et al 2003) to compare with the eddy correlation fluxes. Six sediment cores (20 cm deep and 6 cm diameter) with overlying water and seawater from the site were collected and kept on ice in the dark until returned to the lab.…”

Endogenous versus environmental control of vertical migration by intertidal benthic microalgae

“…Microphytobenthic biofilms play an important role in estuarine ecosystem functioning, contributing to up to 50% of primary production (MacIntyre et al, 1996;Underwood & Kromkamp, 1999). They constitute an essential food source for grazers Middleburg et al, 2000), regulate the exchange of nutrients across the sediment-water interface (MacIntyre et al, 1996;Tyler et al, 2003) and promote sediment biostabilization (Paterson, 1986;Yallop et al, 1994;de Brouwer et al, 2000;Gerbersdorf et al, 2009;Andersen et al, 2010;Stal, 2010).…”

Quantifying photosynthetic rates of microphytobenthos using the triple isotope composition of dissolved oxygen

“…These producers may account for greater than 50% of gross primary productivity, i.e., the total photosynthetic flux, in estuaries (Underwood and Kromkamp 1999). Microphytobenthos have been implicated as a major, and in some cases dominant, controlling factor in estuarine nitrogen cycling (Engelsen et al 2008;Risgaard-Petersen 2003;Tyler et al 2003). Because of their role in moving nitrogen from the overlying water column into the sediments, it is hypothesized that MPB act to buffer estuaries from the effects of eutrophication (McGlathery et al 2004;Meyercordt and Meyer-Reil 1999).…”