Metabolic changes and compositional shifts in nutrient-enriched tropical reef sediment communities

Abstract

Four sets of nutrient enrichment experiments were carried out in 1994 and 1995 using sieved sand from a coral reef flat in the northwestern Philippines. The samples, prepared by packing the sediment in glass petri dishes, were maintained for 7 days in outdoor microcosms with varying nitrogen:phosphorus (N:P) ratios in the overlying water. The objective of the study was to detect changes in the composition and metabolism of sediment communities over short-term nutrient exposures. Net primary production and respiration rates of the microalgae increased within a few days in treatments having elevated levels of N, P and N+P. The addition of N and N+P significantly stimulated microalgal proliferation, even within a few days, while the addition of P alone did not elicit a significant response. Diatoms and cyanobacteria were the major taxonomic groups in the sediment samples, with the latter accounting for much of the community growth. The associated infaunal community in the sediment plates, dominated by crustaceans, nematodes and foraminiferans, showed a high degree of variability between experiments, but this did not confound the effects of nutrient addition as there was no distinct trend with respect to either time or nutrient treatment. This study demonstrates that, in tropical shallow-water benthic systems, the direct effects of an episodic nutrient loading event can be observed within short time scales using microcosms derived from natural communities.