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Phytoplankton growth rates in a light-limited environment, San Francisco Bay

January 1, 1988

Phytoplankton cells reside in a turbulent medium partitioned into an upper photic zone that sustains photosynthesis, and a lower aphotic zone that does not. In estuaries, vertical mixing rates between these 2 zones can be rapid (< 1 generation time) because of tidal stirring and because the mixing depth is generally shallow. Moreover, the photic depth is characteristically shallow in estuaries because of the high seston concentrations that typify these systems (e.g. Cloern 1987). Hence the mean light exposure of phytoplankton cells, and their rates of photosynthesis and growth in estuaries, should be related to the ratio of photic depth Zto mixing Zm, (defined as either the water column depth, or the surface layer depth in a stratified estuary).

Grobbelar (1985) has measured phytoplankton production in turbid waters and used the ratio Zp:Zm as a simple index to quantify the degree of light limitation. Harris and his colleagues have examined in detail the importance of the Zp:Zm ratio in lakes for influencing variability of phytoplankton production and photosynthetic parameters (Harris et al. 1980), and variability of species composition both temporally (Harris & Piccinin 1980) and spatially (Haffner et al. 1980). Phytoplankton growth rates have only rarely been measured in estuaries (Malone 1977, Furnas 1982, Harding et al. 1986) where cycling rates between the photic and aphotic zones can be much faster than in lakes or the open ocean.

This study was motivated by the need for quantitative measures of phytoplankton population growth rate in an estuarine environment, and was designed around the presumption that growth rates can be related empirically to light exposure. We conducted the study in San Francisco Bay (California, USA), which has large horizontal gradients in light availability (Zp:Zm) typical of many coastal plain estuaries, and nutrient concentrations that often exceed those presumed to limit phytoplankton growth (Cloern et al. 1985). We tested the hypothesis that light availability is the primary control of phytoplankton growth, and that previous estimates of growth rate based on the ratio of productivity to biomass (Cloern et al. 1985) are realistic. Specifically, we wanted to verify that growth rate varies spatially along horizontal gradients of light availability indexed as Zp:Zm, such that phytoplankton turnover rate is rapid in shallow clear areas (high Zp:Zm) and slow in deep turbid areas (low Zp:Zm). We used an in situ incubation technique which simulated vertical mixing, and measured both changes in cell number and carbon production as independent estimates of growth rate across a range of Zp:Zm ratios.

Publication Year 1988
Title Phytoplankton growth rates in a light-limited environment, San Francisco Bay
Authors Andrea E. Alpine, James E. Cloern
Publication Type Article
Publication Subtype Journal Article
Series Title Marine Ecology Progress Series
Index ID 70156297
Record Source USGS Publications Warehouse
USGS Organization California Water Science Center; San Francisco Bay-Delta