Topical Area: Algal Blooms

Bill Richardson, Department of Environmental Protection, Florida Marine Research Institute, St. Petersburg, Florida

It has been suggested that the differences in phytoplankton species’ ability to acquire and utilize nutrients may be an important factor in the determination of phytoplankton community composition. As a result, many measurements of uptake and growth kinetic parameters have been made and the outcome of competition experiments predicted from the shapes of the Monod curves. A direct test of phytoplankton resource-based competition is now possible using Tilman’s equilibrium theory of competition. This model uses resource-dependent growth kinetics (Umax (maximal growth rate) and Ku (half saturation constant for growth) to predict a species’ competitive power under nutrient limiting conditions. Under equilibrium conditions, the species with the lowest minimum limiting nutrient requirement at zero net growth rate (R*), will eventually competitively displace all the other species. There is evidence that environmental factors may affect a species’ competitiveness. It has been shown that salinity may alter the ammonia metabolism and temperature may alter the species minimum equilibrium requirement for a limiting nutrient at zero net growth rate (R*).

The influence of salinity on the minimum requirement for a limiting nutrient (R*) is being examined in the following dominant microalgal taxa of Florida Bay; the blue green algae Synechococcus elongatus, a blue green picoplankter, and the diatoms Chaetoceros cf. salsugineus, and Cyclotella choctawhatcheeana. Each species nutrient kinetic parameters, Umax (maximal growth rate) and Ku (half saturation constant for growth) for phosphorus dependent growth is being determined at salinities (S) of 15, 25, and 50 ppt using batch cultures, at 25 0C, 12:12 light:dark cycle and 100 uEm-2sec-1 . The results at S=25 and S=50 were fit to the Monod model. Umax values were found to differ with salinity as predicted from salinity growth curve data. Ku values although having wide confidence intervals, did not differ significantly with salinity. Indicators of competitive ability Umax/ Ku and R* predict that the blue green algae S. elongatus will be by far the best competitor for phosphorous at both S=25 and S=50.The predicted relative competitive ranking under P-limitation is the same for both S=25 and S=50,and in decreasing order of competitiveness is S. elongatus, the blue green picoplankter, C. salsugineus and C. choctawhatcheeana..

Competition experiments are being carried out under approximate steady-state conditions using semi-continuous dilution at S=15 S=25 and S=50 at 25 0C, 12:12 light:dark cycle and 100 uEm-2sec-1 . The outcome of P-limitation experiments will verify the resource competition model predictions and determine interspecific differences in competitive ability when interspecific differences in R* are undetectable. Competition experiments are also being carried out at S=15, S=25 and S=50 under N-limitation and "non-limiting" balanced ratio (N:P,16:1) nutrient conditions. Under each of the above three nutrient regimes (P-limitation, N-limitation and "non-limiting" balanced N:P ratio) are three treatments consisting of (1) a steady supply of the limiting nutrient, (2) a periodic pulse of the limiting nutrient and (3) a steady supply of the limiting nutrient coupled with a repeating oscillating salinity fluctuation (5 ppt decrease followed by a 5 ppt increase, each lasting 5 days). The pulse of the limiting nutrient and the salinity fluctuation will be used to examine the potential role of temporal disturbance on resource competition under P- and N-limitation and "non-limiting" N:P conditions. At S=25, S. elongatus is the dominant competitor for all three treatments under P-limitation, while S. elongatus and the blue green picoplankter were codominants for all three treatments under N-limitation.

Cellular quotas of phosphorus and nitrogen are being determined using batch cultures under the in vitro conditions described above. The minimal phosphorus cell quotas (Q_min) increased with increasing cell size with minimal and maximal values found in S. elongatus and C. salsugineus (.01 and 1.2 fmol/cell respectively). The maximal:minimal phosphorus cell quota ratio (Q/ Q_min) decreased with increasing cell size with maximal and minimal values found in S. elongatus and C. salsugineus (30 and 16 respectively).

Completion of the study investigating the potential role of salinity and nutrient competition in the structuring of the community composition of selected dominant microalgae of Florida Bay as described above is estimated to occur within the year 1998.