Variability in surface water chemistry and phytoplankton biomass in two tropical, tidally dominated mangrove creeks

Abstract

Surface water concentrations of dissolved nutrients and phytoplankton biomass (as chlorophyll a) were examined monthly in relation to physicochemical characteristics and rainfall for 30 months in two tropical, tidally dominated mangrove creeks in north Queensland, Australia. Dissolved nutrient concentrations and phytoplankton biomass peaked during summer with no or little significant change throughout the rest of the year. Dissolved nitrogen and phosphorus concentrations correlated inversely with salinity changes, implying that fresh water and suspended material from the watershed were the main source of dissolved nutrients.The mean dissolved inorganic N : P ratio in each creek (Control Creek 58 : 1, Sandfly Creek 26 : 1) was greater than the Redfield ratio (16 : 1), suggesting excess nitrogen relative to phosphorus. Variability in phytoplankton biomass did not correlate significantly with dissolved nitrogen or phosphorus concentrations, but did relate to rainfall patterns and changes in dissolved oxygen concentrations.These results suggest that pelagic conditions in these mangrove creeks are constant all year round, except during the summer wet season.