Evaluation of benthic nutrient fluxes and their importance in the pelagic nutrient cycles in Suo Nada, Japan

Abstract

The benthic nutrient fluxes in Suo Nada, in the western part of Seto Inland Sea, Japan were estimated seasonally in 2002, and their contribution to the pelagic nutrient cycle was evaluated. The horizontal distributions of inorganic phosphorus (DIP), ammonia and dissolved silica (DSi) concentrations in the pore water of the surface (0-3 cm) sediments were high in the western part, indicating a eutrophic condition relative to the other parts. Seasonally, the highest nutrient concentration in the pore water was observed in summer, as reflected by the increasing decomposition rate of organic matter. Multiple regression analyses revealed significant positive effects of acid volatile sulfide (AVS) and temperature on the DIP flux and also significant positive effects of ignition loss (IL) along with these two parameters on the dissolved inorganic nitrogen (DIN) flux. Temperature, IL and redox potential (Eh) were effective on DSi. The estimated DIP, DIN and DSi fluxes from the sediments were relatively high or equivalent to the total inputs from rains and rivers in annual average, with high values in summer of 2-3-fold higher than the other sources (rain plus rivers). Thus, it is concluded that the benthic nutrient fluxes play an important role in the pelagic nutrient cycle of the Suo Nada ecosystem.

Contribution of benthic microalgae to the whole water algal biomass and primary production in Suo Nada, the Seto Inland Sea, Japan

“…Thus, it is hypothesized that BMA play important roles, not only in primary production but also in the restoration of coastal environments. From the viewpoint of benthic nutrient flux, BMA could reduce the nutrient flux into the water column by absorbing DIP and depressing phytoplanktonic primary production and preventing blooms in the water column, as suggested by Sarker et al, (2005). A similar trend has been reported in other areas (Sundbäck et al, 1991).…”

“…Although the primary production of BMA was not very sensitive to the Ks for DIP ( Figure not shown), uptake of DIP by BMA may be effective in reducing the DIP upward flux from the sediments. Benthic DIP fluxes reported by Sarker et al (2005) in Suo Nada are the reverse of the seasonal pattern of BMA abundance (Fig. 11), suggesting that the assumption holds.…”

“…On the other hand, the higher biomass of BMA in seasons other than summer may reduce the nutrient flux into the water column. An anoxic condition in the warmer season, induced by oxygen consumption by bacterial decomposition in the bottom layer particularly in the southwestern part of Suo Nada (Senjyu et al, 2001), could be responsible for increasing in DIP flux to the water column (Sarker, et al 2005). However, it might be postulated that BMA may partially contribute to controlling benthic nutrient fluxes that consequently control water column primary production, particularly in the southwestern part of the Suo Nada.…”

“…Formation of the oxygen-deficient water is likely due to an increase in the oxygen consumption rate in the bottom layer (Senjyu et al, 2001). Anoxic sediment conditions (acid volatile sulfide ~0.30 mg g -1 dw and redox potential ~-50 mV) in Suo Nada were reported by Sarker et al (2005). Accelerated bacterial decomposition under such conditions helps release nutrients in warmer season, especially in the southwestern part (Sarker et al, 2005).…”

“…Anoxic sediment conditions (acid volatile sulfide ~0.30 mg g -1 dw and redox potential ~-50 mV) in Suo Nada were reported by Sarker et al (2005). Accelerated bacterial decomposition under such conditions helps release nutrients in warmer season, especially in the southwestern part (Sarker et al, 2005). In such an area, BMA inhabiting the surface sediments might play significant roles in the oxygen and nutrient budget by performing primary production.…”