INTEGRATED BIOGEOCHEMICAL OBSERVATIONS OF COASTAL OCEAN PRODUCTIVITY DRIVEN BY STORM INPUTS

Pulsed inputs of storm-runoff to coastal waters of the Hawaiian Archipelago result in rapid changes in coastal water quality and productivity. Our study has focused on the semi-enclosed Kaneohe Bay, Oahu, where long residence times provide for extended reaction between land-derived material and receiving waters. Because storm runoff produces a nutrient enriched surface layer, the persistence of which is dependent on a variety of forcing functions in the bay, understanding the impact of such inputs requires an integrated approach. We have combined continuous in-situ measurements with spatially distributed synoptic sampling to yield data that capture the short-term variability in the system response.

In this presentation we will discuss results obtained at our Coral Reef Instrumented Monitoring Platform (CRIMP) under a variety of environmental conditions ranging from dry background periods to extreme rain events (>25 cm/day) and their aftermath. The CRIMP measures a suite of physical and biogeochemical parameters at 10-minute intervals. Sediment traps on the CRIMP are serviced daily (during storms) to bi-weekly, at which times complementary measurements are also made of physical and biogeochemical parameters at a network of stations distributed throughout the bay.

Large runoff events increased sediment and nutrient loading to bay waters. DIN:DIP ratios of 2 to 4 during background conditions suggest baseline nitrogen limitation of primary productivity. Elevated DIN:DIP (25) in storm runoff changes the proportion of dissolved nutrients available for biological uptake, temporarily relieving N-limitation and often driving the system toward P-limitation. Biological responses include transient increases in Chl-a shortly after storms and longer-term changes in phyto- and zooplankton community structure. Enhancements in primary productivity during storms also affect the CO2 system, temporarily changing bay waters from a net source to neutral or a slight sink of atmospheric CO2. Our approach is particularly well suited to studying ecosystem response over extended periods following pulsed inputs and has allowed us to elucidate the relationships between physical, biological, and chemical processes in the bay, as well as the evolution of plankton community structure subsequent to phytoplankton blooms.