Identification of the environmental variables which determine phytoplankton community structure in two distinct reservoirs

Background/Question/Methods

Phytoplankton community structure is known to change in freshwater lakes, often varying with favorable environmental conditions. Parameters such as water transparency, nutrient concentration, temperature, and rainfall have all been found to impact the phytoplankton community structure in various systems. However, the environmental variables which influence phytoplankton community structure in Puerto Rican reservoirs are largely unknown. In fact, the only studies examining phytoplankton within these reservoirs have focused on species identification. Thus, there is a need to relate the occurrence of groups within the phytoplankton community in Puerto Rico to environmental variables. The objective of this study was to determine whether differences in the spatial and temporal phytoplankton community composition could be explained by environmental parameters for two subtropical Puerto Rican reservoirs, which differed in initial nutrient status. The two reservoirs were sampled during a two year period in the dry and wet seasons. Each reservoir was divided into three separate zones: dam, transition, and riverine. 6 samples were taken per zone. Data collected from each sample point included: temperature, secchi disc depth, pH, TP, TKN, chlorophyll a, and phytoplankton community structure. In order to relate environmental parameters with the phytoplankton community structure, a canonical correspondence analysis (CCA) was conducted.

Results/Conclusions

Secchi disc depth and chlorophyll a concentration were found to vary spatially in both reservoirs during nearly all sampling periods. Spatial differences were expected within the reservoirs due to the mouth of a river entering in one zone. However, only in the eutrophic reservoir did TP, TKN, and phytoplankton biomass differ spatially. Though total biomass did fluctuate spatially, the biomass of phytoplankton groups did change through time. Dinophyceae was found to dominate both reservoirs in the wet season, while Chlorophyceae tended to be the most abundant in the dry season. This trend suggests that phytoplankton in Puerto Rican reservoirs also follow the seasonal succession pattern observed in many freshwater lakes. Each reservoir underwent a separate CCA. Temperature, pH, and TP were found to explain 37% of the variance observed in the mesotrophic reservoir, while specific conductance, pH, NO₃, and TP explained 77% of the variance in the eutrophic system. These results suggest the most influential environmental variables on the phytoplankton community vary with the trophic status of the system. Although there were differences spatially within both reservoirs, temporal patterns seemed to be the most influential in determining phytoplankton community composition.