Interactions of water levels with water quality, endemic waterbirds, and invasive species in a shallow, tropical pond

Abstract

This study responds to a need for baseline physical and chemical data in shallow tropical water bodies. We investigated the degree to which water level fluctuations affected water quality and biota in a shallow tropical pond, Keālia Pond National Wildlife Refuge in Hawaii. We observed strong interactions among climate, water levels, salinity, DO, turbidity, waterbirds, fish, and invertebrates. Seasonal precipitation and air temperature patterns led to higher pond levels in winter and lower pond levels in summer. Low water levels in summer were associated with very high salinity and turbidity, and low DO. Endemic waterbirds were favored by lower water levels. Extremely low DO concentrations at minimum water levels caused fish die-offs, primarily non-native tilapia. The fish die-offs temporarily resulted in lower turbidities, clearer water, and a large increase in invertebrates, specifically a native rotifer but also a non-native chironomid midge. Nuisance emergences of midge only occurred in winters following summer fish die-offs but only if there was sufficient winter streamflow to lower salinity and create brackish conditions in the pond. The study results were used to assess restoration and management options to promote endemic waterbirds and other natives and suppress invasive species.