Temporal and spatial variations of nutrients in the Ten Mile Creek of South Florida, USA and effects on phytoplankton biomass

Water quality throughout south Florida has been a major concern for many years. Nutrient enrichment in the Indian River Lagoon (IRL) is a major surface water issue and is suggested as a possible cause of symptoms of ecological degradation. In 2005-06, water samples were collected weekly from seven sites along Ten Mile Creek (TMC), which drains into the Indian River Lagoon, to investigate and analyze spatial and temporal fluctuations of nutrients nitrogen (N) and phosphorus (P). The objective of this study was to understand the relationships among chlorophyll a concentration, nutrient enrichment and hydrological parameters in the surface water body. High median concentrations of total P (TP, 0.272 mg L-1), PO4-P (0.122 mg L-1), and dissolved total P (DTP, 0.179 mg L-1); and total N(TN, 0.988 mg L-1), NO3-- N (0.104 mg L-1), NH4+-N (0.103 mg L-1), and total Kjeldahl N (TKN, 0.829 mg L-1), were measured in TMC. The concentrations of TP, PO4-P, DTP, TN, NO3--N, NH4+-N, and TKN were higher in summer and fall than in winter and spring. However, chlorophyll a and pheophytin concentrations during this period in TMC varied in the range of 0.000-60.7 and 0.000-17.4 mg L-1, with their median values of 3.54 and 3.02 mg L-1, respectively. The greatest mean chlorophyll a (10.3 mg L-1) and pheophytin (5.71 mg L-1) concentrations occurred in spring, while the lowest chlorophyll a (1.49 mg L-1) and pheophytin (1.97 mu g L-1) in fall. High concentrations of PO4-P (>0.16 mg L-1), DTP (> 0.24 mg L-1), NO3--N (> 0.15 mg L-1), NH4+-N (> 0.12 mg L-1), and TKN (> 0.96 mg L-1), occurred in the upstream of TMC, while high concentrations of chlorophyll a (> 6.8 mg L-1) and pheophytin (> 3.9 mg L-1) were detected in the downstream of TMC. The highest chlorophyll a (11.8 mg L-1) and pheophytin (6.06 mg L-1) concentrations, however, were associated with static and open water conditions. Hydrological parameters (total dissolved solid, electrical conductivity, salinity, pH, and water temperature) were positively correlated with chlorophyll a and pheophytin concentrations (P < 0.01) and these factors overshadowed the relationships between N and P concentrations and chlorophyll a under field conditions. Principal component analysis and the ratios of DIN/DP and TN/TP in the water suggest that N is the limiting nutrient factor for phytoplankton growth in the TMC and elevated N relative to P is beneficial to the growth of phytoplankton, which is supported by laboratory culture experiments under controlled conditions.