Periphyton growth on submerged artificial substrate as a predictor of phytoplankton response to nutrient enrichment

Abstract

Net-pen aquaculture provides the large quantities of fish needed to supplement an increasing consumer demand, but it is also a source of nutrient enrichment to the surrounding ecosystem. Nutrient enrichment can result in measurable increases in phytoplankton growth, but in flowing water, such measurements are difficult and may not be appropriate. An increased awareness of the link between hypernutrification and increased prevalence of nuisance algal blooms heighten this concern. We present a method based on standard lipidology techniques that uses periphyton growth as an indicator of phytoplankton response to nutrient enrichment. Chlorophyll a (Chl a) measurements of phototrophic biomass were used to determine phytoplankton and periphyton response to nutrient stimuli and periphyton growth rates at field sites near net-pen aquaculture facilities. Laboratory trials showed that phytoplankton and periphyton communities responded similarly to both nutrient-sufficient and nutrient-limiting conditions. When sufficient nutrients were present, both phytoplankton and periphyton biomass did not increase during two-day incubations. The addition of plant nutrients during periods of nutrient limitation induced increases of 122 to 175% in phytoplankton biomass and 139 to 221% in periphyton biomass. The similarity in response suggests that the periphyton response would be an accurate descriptor of phytoplankton response. Preliminary field studies showed that reproducible periphyton growth rates (coefficient of variation <10%) were obtained at growth rates greater than 20.0 ng Chl a cm −2 day −1. Periphyton growth rates ranged from 6.0 to 80.0 ng Chl a cm −2 day −1, and were locally enhanced (2.5× that of ambient station) down-current of a freshwater net-pen.