Seasonal pattern and inferred phosphorus limitation of phytoplankton biomass in two tropical reservoirs in northern Australia

Abstract

Manton River Reservoir (MRR) and Darwin River Reservoir (DRR) are two small impoundments in the Australian wet/dry tropics. Over an eight-year period, chlorophyll a concentrations in the mixed layer averaged 3.6 µg L⁻¹ in DRR, and 7.1 µg L⁻¹ in MRR. The seasonal pattern of chlorophyll a at MRR was influenced by wet season wash-out (February average 4.8 µg L⁻¹ ), and dry season destratification and nutrient enrichment of the surface waters (July average 8.4 mg L⁻¹ ). In contrast, DRR exhibited near uniform chlorophyll a concentrations over the year. The seasonal patterns of DRR and MRR chlorophyll a are typical of tropical water bodies which tend to have a smaller annual range than temperate lakes, though this can be modified by significant wash-out. Empirical evidence suggests that the phytoplankton biomass of each reservoir is phosphorus limited, relative to the potential provided by other nutrients and light energy. This conclusion is based on a regression of total phosphorus and chlorophyll a concentrations of pooled DRR and MRR data (P < 0.001; r² = 0.90), and the high total-nitrogen to total-phosphorus concentration ratios (by weight) of 50 and 37 in DRR and MRR, respectively. Annual chlorophyll a and total phosphorus concentrations for both reservoirs are in accord with the OECD regression for temperate lakes and reservoirs.