Environmental influence on phytoplankton production during summer on the KwaZulu-Natal shelf of the Agulhas ecosystem

Abstract

During February 2010, studies of primary production (PP) and physiology were conducted at five selected sites in the KwaZulu-Natal (KZN) Bight of the Agulhas ecosystem as part of a programme to elucidate the influence of major physical driving forces and nutrient inputs on the structure and functioning of biological communities. These sites were located in the vicinity of the Durban lee eddy, in the midshelf region of the central part of the bight, off the Thukela Mouth, and to the north and south of Richards Bay. At four of the sites, chlorophyll a ranged from 0.10 to 1.44 mg m^–3 and integrated PP ranged between 0.35 and 2.58 g C m^–2 d^–1. The highest biomass and PP, which were comparable to those observed in a wind-driven upwelling system, were associated with a diatom community observed at the midshelf site, and varied between 0.26 and 4.27 mg m–3 and 7.22 and 9.89 g C m^–2 d^–1, respectively. Environmental conditions at each of the sites differed substantially and appeared to be influential in initiating and controlling the development and distribution of phytoplankton biomass and production. Phytoplankton adaptation to variable environmental conditions was characterised by a decreased light-limited slope (α^B) and increased rate of photosynthesis (P_m^B) and light saturation (E_k) with elevated temperatures. The converse (increased α^B and decreased P_m^B and E_k) was observed as irradiance levels declined. Generalised additive models indicated that irradiance, temperature and biomass were important variables influencing photosynthetic parameters and photosynthetic rates.

Keywords: generalised additive models (GAMs), KwaZulu-Natal Bight, phytoplankton physiology, primary production