|Nutrient cycling and the pelagic food web in the Oosterschelde – Determining the role of Pacific oyster reefs
Mazzoli, A. (2020). Nutrient cycling and the pelagic food web in the Oosterschelde – Determining the role of Pacific oyster reefs. MSc Thesis. NIOZ Royal Netherlands Institute for Sea Research: Yerseke. 47 pp.
A mesocosm experiment was performed for 185 hours in May 2019 using four densities (zero, two, six and 12 individuals per tank) of Pacific oysters (Crassostrea gigas), an invasive species in the Oosterschelde. 13C-DIC was added to the four tanks and the incorporation of 13C in phytoplankton, bacteria, micro- and mesozooplankton, oysters and detritus was analyzed over time and used to formulate a mathematical model for the investigation of trophic interactions. Oysters exerted a topdown control on the system through seston filtration, as they efficiently removed phytoplankton and zooplankton, but simultaneously they contributed to nitrogen regeneration, mainly as ammonium. These processes were enhanced as oyster density increased, supporting the role of C. gigas in the intensification of the benthic-pelagic coupling. Retention of silica and limited release of phosphate by C. gigas were detected with three shorter microcosm experiments (2-5 hours), where individual oysters were incubated. Nutrient replenishment only led to a limited phytoplankton growth due to the strong pressure exerted by oysters, which altered the phytoplankton community due to preference of specific taxa (diatoms over chlorophyta) and size. The latter was dictated by probable proliferation of picophytoplankton towards the end of the experiment due to inefficient retention of small organisms (< 3 μm) by oysters. Mesozooplankton were readily removed at high oyster densities (six and 12 oysters), acting as an additional food resource to meet energy requirements; larvae were preferentially filtered at low oyster density (two oysters), as adult mesozooplankton might hinder digestion due to its size. In the latter system, the co-presence of oysters and considerable mesozooplankton abundance led to comparable predation pressures on lower trophic levels. This was corroborated by clearance rates and model outputs, which placed the four mesocosm systems in one of two states (zero and two oysters vs. six and 12 oysters). Slower 13C incorporation in DOC, bacteria, mesozooplankton and oysters were detected at higher oyster densities (six and 12 oysters) due to the severe oyster filtration on all lower trophic levels. The high clearance rate of C. gigas has been previously reported and from our study its role as top predator via efficient filtration was confirmed at high oyster densities. This benthicdominated system is representative of the Oosterschelde, where C. gigas currently dominates intertidal flats. Only the eastern basin is likely to be pelagic-dominated due to a severe reduction in oyster reefs and a proliferation of mesozooplankton (comparable to our tank with only two oysters).