Determining Chlorophyll-a degradation and its accuracy as a particle-tracer for bioturbation compared to luminophores

Abstract

By mixing sediment particles, marine bioturbating animals have an essential influence on ecosystem functioning. To estimate how animals affect ecosystem functions, it is necessary to test widely used methods for accuracy and improve bioturbation quantification methods. In this study Chlorophyll-a (Chl-a) and luminophores were used as particle-tracers to quantify bioturbation rates (Db) in muddy and sandy sediments of the Dutch Eastern Scheldt. The muddy site (Zandkreek) held 7.45 times more biomass and 3.64 times higher abundance compared to the sandy site (Dortsman). Luminophore- based bioturbation rates corresponded well with sediment oxygen consumption and the biological trait derived Bioturbation Potential index (BPc), which were all significantly higher in muddy Zandkreek. In contrast, estimated Chl-a-based bioturbation rates measured were not significantly different between sediment types and corresponded poorly with BPc and oxygen consumption. Also, in both sediment types Chl-a-based bioturbation rates were an order of magnitude higher than the luminophore-based bioturbation rates. Anaerobic Chl-a degradation (kd) was 50% and 25% lower than aerobic Chl-a degradation in sandy and muddy sites respectfully, but the large confidence interval highlights the variability in kd. Furthermore, kd values in this study were an order of magnitude smaller compared to kd values previously determined in other bioturbation studies. The variability in Chl-a degradation causes uncertainty in Chl-a-based bioturbation rates, owing to the linear scaling between the two variables in biodiffusive mixing models. Because Chl-a degradation varies under a range of environmental (redox) conditions, the use of Chl-a as a particle tracer in bioturbation quantification should, therefore, be considered with caution.