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 variability 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 April 1994 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, multiplying the species-specific excretion rates by the actual animal biomass found on the tidal flat. Additionally, for all different seasons, we calculated the upward diffusive flux from the sediments from the vertical profiles of nutrient concentrations in the porewater. On the flat, the bivalves Ruditapes philippinarum (Veneridae) and Musculista senhousia (Mytilidae) were dominant, making up 86 ± 5.6% when the total biomass exceeded 100 g DW (dry weight) m-2. From our laboratory experiments, the mean (day and night) nutrient excretion rates at 20 to 22°C by the 2 bivalve species were 18.9 μmol NH4+-N g-1 DW h-1, 4.8 μmol (NO3- + NO2-)-N g-1 DW h-1 and 3.3 μmol PO43--P g-1 DW h-1. In addition, NH4+-N excretion by R. philippinarum, but not that by M. senhousia, was significantly higher during the day than during the night. This occurred while the food (Thalassiosira sp.) offered in spikes was rapidly taken up irrespective of the concentration (within a field relevant spring-summer range of 10 to 60 μg l-1 chl a) and in 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-2 d-1, 8.8 mmol (NO3- + NO2-)-N m-2 d-1 and 5.8 mmol PO43--P m-2 d-1. 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-2) of the most investigated mussel, Mytilus edulis. 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-2 d-1 and 0.01 to 0.05 mmol PO43--P m-2 d-1, thus more than 1 order of magnitude lower than that due to the excretory activity by R. philippinarum and M. senhousia. Our results indicate that the dominant bivalves, R. philippinarum and M. senhousia, play a major role in the processes of benthic nutrient regeneration within the intertidal zone, rapidly and efficiently recycling the inorganic forms of N and P available to primary producers. This study highlighted the importance of considering the temporal scaling of intertidal macrozoobenthos distribution in the quantification of the processes of benthic nutrient regeneration in these highly variable systems.