Light and nutrient limitation on phytoplankton production in the strait of an enclosed coastal sea (Bisan Strait, eastern Seto Inland Sea, Japan)

- The Bisan Strait (BS) is a relatively turbid shallow area (Kd = 0.4 m− 1, depth = 14 m).
- P-I curves and phytoplankton production were examined in the BS.
- Photosynthetic activity peaked in autumn because of increased nutrient and temperature.
- Production did not peak in autumn because of low light availability.
- Light explained differences in annual production between the BS and adjacent areas.

The Bisan Strait is a vertically well-mixed, shallow area (mean depth 13.9 m) in the Seto Inland Sea. The strait has the lowest Secchi transparency (mean 4.5 m) within the Inland Sea because of active sediment re-suspension. Therefore, in comparison with adjacent areas, phytoplankton production in the strait may be strongly affected by light availability in addition to nutrient availability. In this study, we examined environmental variables, photosynthesis-irradiance (P-I) curves and phytoplankton production in the Bisan Strait over 1 year. There were temporal variations in the light-saturated photosynthesis rate (PBm) and initial slope of P-I curve (αB), with maxima in autumn and minima in spring. Most of the variability in PBm and αB was explained by variations in nutrient concentrations (dissolved inorganic nitrogen) and water temperature. Meanwhile, phytoplankton production reached a peak in summer and a nadir in spring, but an autumn peak in production was not observed. Diagnostic analysis suggested that, for almost all of the year, nutrients were more important for phytoplankton growth than light limitation. However, light limitation became more important in autumn when underwater irradiance reached low levels. Therefore, the lack of an autumn peak in production is likely to be related to light limitation. We suggest that low light availability during the autumn depresses the annual rate of phytoplankton production in the Bisan Strait, in comparison with adjacent areas where seasonal stratification is established and phytoplankton blooms frequently occur in early autumn.