Nutrient Inputs, Phytoplankton Response, and CO2 Variations in a Semi-Enclosed Subtropical Embayment, Kaneohe Bay, Hawaii

• Published in: Aquatic geochemistry 2011-09, Vol.17 (4-5), p.473-498
• Main Authors: Drupp, Patrick, De Carlo, Eric Heinen, Mackenzie, Fred T., Bienfang, Paul, Sabine, Christopher L.
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric forcing; Biogeochemistry; Carbon dioxide; Careers; Chemical analysis; Climatic zones; Coastal zone; Coral reef ecosystems; Coral reefs; Earth and Environmental Science; Earth Sciences; Geochemistry; Groundwater discharge; Hydrogeology; Hydrology/Water Resources; Marine ecosystems; Monitoring systems; Mountain regions; Mountains; Nutrients; Original Paper; Phytoplankton; Plankton; River flow; Rivers; Runoff; Seawater; Sediment transport; Storms; Streams; Surface water; Tropical environments; Water analysis; Water quality; Water Quality/Water Pollution
• ISSN: 1380-6165; 1573-1421
• DOI: 10.1007/s10498-010-9115-y

The marine shelf areas in subtropical and tropical regions represent only 35% of the total shelf areas globally, but receive a disproportionately large amount of water (65%) and sediment (58%) discharges that enter such environments. Small rivers and/or streams that drain the mountainous areas in these climatic zones deliver the majority of the sediment and nutrient inputs to these narrow shelf environments; such inputs often occur as discrete, episodic introductions associated with storm events. To gain insight into the linked biogeochemical behavior of subtropical/tropical mountainous watershed-coastal ocean ecosystems, this work describes the use of a buoy system to monitor autonomously water quality responses to land-derived nutrient inputs and physical forcing associated with local storm events in the coastal ocean of southern Kaneohe Bay, Oahu, Hawaii, USA. The data represent 2.5 years of near-real time observations at a fixed station, collected concurrently with spatially distributed synoptic sampling over larger sections of Kaneohe Bay. Storm events cause most of the fluvial nutrient, particulate, and dissolved organic carbon inputs to Kaneohe Bay. Nutrient loadings from direct rainfall and/or terrestrial runoff produce an immediate increase in the N:P ratio of bay waters up to values of 48 and drive phytoplankton biomass growth. Rapid uptake of such nutrient subsidies by phytoplankton causes rapid declines of N levels, return to N-limited conditions, and subsequent decline of phytoplankton biomass over timescales ranging from a few days to several weeks, depending on conditions and proximity to the sources of runoff. The enhanced productivity may promote the drawing down of pCO 2 and lowering of surface water column carbonate saturation states, and in some events, a temporary shift from N to P limitation. The productivity-driven CO 2 drawdown may temporarily lead to air-to-sea transfer of atmospheric CO 2 in a system that is on an annual basis a source of CO 2 to the atmosphere due to calcification and perhaps heterotrophy. Storms may also strongly affect proximal coastal zone pCO 2 and hence carbonate saturation state due to river runoff flushing out high pCO 2 soil and ground waters. Mixing of the CO 2 -charged water with seawater causes a salting out effect that releases CO 2 to the atmosphere. Many subtropical and tropical systems throughout the Pacific region are similar to Kaneohe Bay, and our work provides an important indication of the var