Temporal scaling and relevance of bivalve nutrient excretion on a tidal flat of the Seto Inland Sea, Japan

Abstract

Using an indirect and integrated approach, we quantified the magnitude and temporal variabiiity of the contribution of macrozoobenthos to the upward flux of inorganic nitrogen and phosphorus on a sandy tidal flat of the Seto inland Sea, Japan. From Apnl1994 to April 1996, we conducted monthly field surveys on the abundance and faunal composition of the macrozoobenthic communities inhabiting the lower part of the intertidal zone. Subsequently, we carried out 24 h day/night laboratory experiments on the nutrient excretion rate by various size-classes of the dominant species. We then obtained the animal nutrient excretion over a 2 yr period, muitiplying the species-specific excretion rates by the actual animal biomass found on the tidal flat. Additionaliy, for ali different seasons, we calcuiated the upward diffusive flux from the sediments from the vertical profiies of nutrient concentrations in the porewater. On the flat, the bivalves Ruditapes phihppinarum (Veneridae) and Musculista senhousia (Myhhdae) were dominant, making up 86 I 5.6 % when the total biomass exceeded 100 g DW (dry weight) m-'. From our laboratory experiments, the mean (day and night) nutrient excretion rates at 20 to 22OC by the 2 bivalve species were 18.9 p n o l NH4+-N g-' DW h-I, 4.8 pnol (NO3-+ NO2-)-N g-' DW h-' and 3.3 pmol P043--P g-' DW h-I. ln addition, NH4+-N excretion by R. phiiippinarum, but not that by M. senhousia, was significantly higher during the day than during the night. This occurred while the food (Thalassiosua sp.) offered in spikes was rapidly taken up irrespective of the concentration (within a field relevant spring-summer range of 10 to 60 pg T' chl a) andin day/night treatments. The release of nitrogen (N) and the release of phosphorus (P) were highly correlated with each other, for both R. philippinarum and M. senhousia, with a N/P ratio of 7.8 ? 3.0 and 9.9 * 3.5, respectively. In the field, the bivalve excretion rates of nutrients were calculated to be up to 35.2 mmol NH4+-N m-' d-I, 8.8 mrnol (NO3-+ NO2-)-N m-' d-' and 5.8 mmol P O~~--P m-' d-'. These values rank in the upper range of nutrient excretion by intertidal macrozoobenthos and are comparable to those found on dense assemblages (800 to 2000 g ash free DW m-') of the most investigated mussel, Mytzius eduiis. In addition, nutrient fluxes through bivalve excretion varied strongly within a few months, up to 10-fold (R. philippinarum) and 100-fold (M. senhousia) between April 1994 and August 1994, as related to the temporal change of bivalve standing stock. The extent of nutrient regeneration through diffusive flux was comparable to that reported for other intertidal zones of Japan and in the eutrophic Seto Inland Sea, within a range of 0.2 to 1.5 mmol NH4+-N m-' d-' and 0.01 to 0.05 mmol P043--P m-' d-I, thus more than 1 order of magnitude lower than that due to the excretory activity by R. phiiippinarum and M. senhousia. Our resuits indicate that the dominant bivalves, R. phihppinarum and M. senhousia, play a major role in the processes of benthic...

Benthic primary production and nutrient cycling in sediments with benthic microalgae and transient accumulation of macroalgae

“…Also, the infauna type plays an important role in regulating the balance between mineralization and nutrient release. Bivalves, which made up 93% of the infauna biomass at Virksund, excrete the NH produced by mineralization via ϩ 4 the siphons directly to the overlying water (Magni et al 2000). The benthic microalgae therefore did not have the for just a couple of months during summer completely changed both the nutrient and oxygen budgets of these shallow-water sites.…”

Sponges as a source of dissolved inorganic nitrogen: Nitrification mediated by temperate sponges

“…Furthermore, nutrient fluxes at the sediment-water interface can influence or regulate the nutrient composition of the water column, and benthos behave as either sinks or sources of inorganic nitrogen (N), phosphorus (P), and silicon (Nixon 1981). In some cases, nutrient remineralization mediated by macrozoobenthos (e.g., Magni et al 2000) and in the sediment can supply a large proportion of autotrophic nutrient demand (Middelburg and Soetaert 2005). However, despite the current understanding of nutrient dynamics in estuaries, the role of benthic suspension feeders dominating rocky bottom habitats in nutrient dynamics has received little attention.…”

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

“…Bartoli et al (2001) showed a linear increase in the NH 4 -N flux to the water column with increasing biomass of the clam R. philippinarum. Magni et al (2000) reported that the nutrient mineralization rate of infaunal bivalves (R. philippinarum and M. senhousia) was between 16 and 82 times higher than the diffusive flux from the sediment of tidal flats. Therefore, in this study, mineralization by the clam seemed to be a major source of DIN in the water column in LTHS.…”

“…Asmus and Asmus (1991) also suggest that the filtration activity of the mussel Mytilus edulis reduces standing stocks of phytoplankton, and that nutrient mineralization sustains primary phytoplankton production. Nutrient mineralization by infaunal bivalve species has been studied in laboratory and mesocosm experiments (Doering et al, 1987), in situ benthic chambers, sediment core incubations (Cockcroft, 1990;Yamamuro and Koike, 1993;Bartoli et al, 2001), and by quantification of the total upward flux of nutrients from sediments and subsequent estimation of the relative contribution of animal excretion (Nakamura et al, 1988;Kiibus and Kautsky, 1996;Magni et al, 2000;Hiwatari et al, 2002;Kohata et al, 2003). These studies have shown that nutrient mineralization is an important source for water-column primary production, and this concept is incorporated in the ecological model for infaunal bivalve fisheries (e.g., Zaldívar et al, 2003;Spillman et al, 2008).…”