Photosynthesis/light relations in the shoals of San Francisco Bay

Summary

The San Francisco Bay receives large inputs of nutrients from domestic, industrial and agricultural sources, and yet shows little sign of eutrophication. Both light availability and photoinhibition have been considered by local experts as factors which may limit primary production in this very shallow estuary.

Photosynthesis/light relations were studied in situ from April to August 1982 using the ¹⁴C method. The euphotic zone in the shoals, based on underwater light measurements, is approximately 4 m. Despite high levels of turbidity, photoinhibition was observed in the surface layer above 0.5 m. Both maximum rate of photosynthesis and maximum photosynthetic efficiency occurred at relatively high light intensities, averaging 360 and 335 W · m⁻², respectively. Thus, the spring and summer phytoplankton populations showed adaption to strong light intensities, as might be expected in a shallow, well-mixed system. In the shoals, where the photic zone is truncated by shallow depth, increased light limitation is predicted to have a greater effect than increased photoinhibition in reducing integral photosynthesis. Photoinhibition decreased the depth integral of photosynthesis by less than 5%, and is unlikely to be an important factor limiting primary production.