Oceanic nutrient supply and uptake by microphytobenthos of the Hichirippu Lagoon, Hokkaido, Japan

摘要

We conducted field surveys to determine spatio-temporal variations in the water quality (including inorganic nutrient concentration) of a coastal lagoon, Hichirippu Lagoon (3.56 km~2), in Hokkaido, Japan. During a typical supply period (winter), a 24 h continuous survey was conducted to evaluate nutrient supply from the open ocean and examine nutrient utilization by microphytobenthos (MPB) based on the mass balance of a biophilic element. During February 2008, the majority of NO_3+NO_2-N was supplied from the open ocean, and the dissolved inorganic nitrogen (DIN) budget was 3.25 kmol N d~(-1). In March 2008, the total amount of organic nitrogen was 39.0 to 588 kmol N for MPB in the surface sediment (depth, 0.5 cm). Assuming that MPB in the surface sediment used all NO_3+NO_2-N input (3.93 kmol N d~(-1)), their temporal division rate (NO_3+NO_2-N uptake/total amount of MPB) would be 0.01 to 0.10 d~(-1). Here the annual DIN budget was estimated to be 258 kmol N yr~(-1), which mainly depended on MPB-fixed oceanic DIN during winter.