Nitrogen fluxes, denitrification and the role of microphytobenthos in microtidal shallow-water sediments:an annual study

Abstract

The annual cycle of oxygen and nitrogen flux, denitrification and microphytobenthic variables (primary production, biomass, composition and calculated N demand) were studied for 2 shallow-water microtidal sediment sites in NE Kattegat, 1 sandy and 1 silty, by incubating undisturbed sediment in the laboratory in light and darkness. Both sites (2 stations within each) were characterised by low concentrations of inorganic nitrogen (IN), in the overlying water during summer (NO3 generally < l pM), with winter concentrations of 5 and 30 pM for the sandy and silty site, respectively. Through the activity of microphytobenthos, the sediment systems appeared to be net autotrophic during most of the year. Net oxygen production varied between -400 and 6600 pm01 m-? h-', being highest during the warmest season. Although the composition of the microphytobenthos depended on the sediment type, it did not have a crucial effect on the magnitude of the microphytobenthic biomass or function. The .temporal pattern of the function of the microalgal community, on the other hand, was significantly influenced by the sediment type; sandy sediment exhibited a smooth seasonality, controlled mainly by temperature and light, while the silty microtidal sediment was also controlled by stochastic events, such as sediment resuspension. Microphytobenthos had a significant effect on the IN flux, the clearest effect being found for NH4. Total denitrification (isotope-pairing technique) generally varied between < l and 40 pm01 m-2 h-', being dominated by nitrification-coupled denitrification (D,,), and being 1 order of magnitude higher at the silty site. Microphytobenthic activity generally inhibited denitrification in the low-N areas in this study. The results suggest that the microphytobenthos functions as a major control throughout the annual cycle, by forming communities that are net photoautotrophic throughout the year, and by significantly influencing both the IN flux and denitrification rates. Sandy sediment appeared to function as an IN sink during winter and early spring, while no clear seasonal pattern was found for silty sediment. Calculated N demand of the microphytobenthos far exceeded the measured sediment net uptake of N, supporting the idea that sandy systems in low-nitrogen areas can be highly productive through a closed recycling of N. The ratio between calculated microphytobenthic N demand and measured denitrification rates suggests that denitrification has a minor role as a N sink, particularly in sandy, cold-climate microtidal sediments. MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…In general this is expected since denitrification in marine sediments is a temperature dependent process (Rysgaard et al 2004). On the other hand, our finding is in contrast with other studies from temperate estuaries and coastal systems, where lower denitrification rates were observed in summer during limiting NO 3 -and O 2 conditions Cabrita and Brotas 2000;Sundbäck et al 2000). At B1, H2 and H4, however, O 2 was never limiting and NO 3 -for denitrifiers was mainly generated through nitrification.…”

Reevaluation of the nutrient mineralization process by infaunal bivalves (Ruditapes philippinarum) in a shallow lagoon in Hokkaido, Japan

“…Microphytobenthos take up nutrients not only from the water column but also from pore water (Henriksen et al, 1980;Sundbäck et al, 1991;Cahoon and Cooke, 1992;Cahoon, 1999;Sundbäck et al, 2000;Srithongouthai et al, 2003). In this study, the mean standing stock of microphytobenthos inhabiting surface sediment (5 mm thick) on tidal flats was 100 times higher than that of phytoplankton (1 m depth; see section 3.3).…”

“…Previously, in situ measurements using light and dark chambers on tidal flats have shown that the efflux of NH 4 -N from the sediment depends on the photosynthetic activity of microphytobenthos (Sundbäck et al, 1991;Sundbäck et al, 2000;Sandwell et al, 2009). In the continuous survey of this study, the factor loading of NH 4 -N was significantly negative but lower than the other nutrients, it seemed to reflect the effect of both physical variables (i.e., tidal height) and photosynthetic activity (i.e., solar radiation) ( Table 1, Fig.…”

“…Microphytobenthos take up nutrients from both the water column and pore water (Henriksen et al, 1980;Sundbäck et al, 1991;Cahoon and Cooke, 1992;Cahoon, 1999;Sundbäck et al, 2000;Srithongouthai et al, 2003). Even though it is easy to assume that there is an important relationship between microphytobenthos and infaunal bivalve species, this relationship can only be derived from laboratory and in situ measurement studies (e.g., Swanberg, 1991;Sandwell et al, 2009).…”

Benthic fluxes of oxygen and nutrients in sublittoral fine sands in a north-western Mediterranean coastal area

“…GPP was expressed in carbon units applying photosynthetic quotient O 2 :C=1.2 (Bartoli et al, 2012) in order to estimate theoretical N, P, and Si requirements by microphytobenthos (Table 6). Theoretical nutrient assimilations were obtained from the ratio C:N=9 as employed by Sundbäck et al (2000) and Alsterberg et al (2012) for microphytobenthos and from Redfield stoichiometric ratios (C:P=106 and C:Si=6.6). To calculate benthic regeneration contribution to microphytobenthos, theoretical nutrient demand was compared to benthic nutrient fluxes in dark conditions.…”