Temporal and spatial scaling of planktonic responses to nutrient inputs into a subtropical embayment

We carried out a study of the spatial and temporal effects of land-derived material on water column nutrients and plankton dynamics in a subtropical estuary. The study had 2 parts: (1) a 3 yr synoptic monitoring program, and (2) a shorter 1.5 yr study during the second half of the program, which focused on individual pulses driven by discrete rainfall events. Although we found spatial differences in some water column parameters within Kāne‘ohe Bay and an adjacent oceanic site, inorganic nutrient levels were generally comparable in the Bay and offshore. One difference was that Prochlorococcus spp. numerically dominated the plankton at the oceanic site whereas Synechococcus spp. dominated at all Bay sites. The switch in dominance appears to be due to light characteristics and dissolved organic nitrogen (DON), but not dissolved inorganic nutrient availability. There were no annual cycles in water column parameters within the Bay; however, a comparison of dry and wet seasons did show some differences. Planktonic cell abundance was in general lower during the wet season, with the exception of opportunistic diatoms that were more abundant during the wet season. A drought during the study period may have influenced our results. Pulses were characterized by an elevation in inorganic nutrient concentrations in the Bay close to the stream mouth. The general response was an increase in abundance of microphytoplankton and chl a after a 3 to 6 d lag following the nutrient increase. Picophytoplankton showed an increase in fluorescence per cell after a 12 to 24 h lag, probably related to a decrease in irradiance associated with turbidity in runoff. The Bay can act as source of dissolved inorganic nutrients and plankton for oceanic waters; however, planktonic populations in the Bay are primarily autochthonous and do not represent an oceanic source of nutrients for plankton consumers within Kāne‘ohe Bay.