Temperature and salinity as abiotic drivers of zooplankton community dynamics in the Great Berg Estuary

Abstract

The zooplankton of the Great Berg Estuary was sampled between February 2003 and February 2006 at 15 stations on seven occasions. The estuary experiences river dominance in winter and marine dominance in summer, leading to strong seasonal shifts in salinity distribution along the estuary. Despite river-dominance, the three major groups of zooplankton (a euryhaline marine assemblage, a euryhaline assemblage and a oligohaline associated assemblage) were identified in the zooplankton. Pseudodiaptomus hessei was numerically the most important species in the estuary, contributing over 65% to total copepod abundance on average. Maximum density exceeded 50 000 m⁻³ of water, comparable to maximum levels of abundance recorded in many other South African estuaries. Copepods as a group contributed between 47 and 99% to total abundance of all zooplankton species (49), but density of most of these was comparatively low. Salinity gradients were not correlated to abundance and distribution of P. hessei along the estuary, further illustrating the ability of the species to exploit a wide salinity range. However, salinity consistently emerged as the most important variable that explained most of the correlation (67–94%) at the zooplankton community level. Water temperatures in the middle and upper Great Berg Estuary were consistently warmer compared to lower estuarine reaches in summer when the difference was 6.2°C on average. A comparison of water temperatures in middle and upper estuarine reaches in five permanently tidal estuaries between the Olifants Estuary on the west coast and the Mngazana Estuary on the east coast reflected a difference of ca 7–8°C in summer and 5–6°C in winter across the biogeographical provinces. The elevated water temperatures in middle and upper estuarine reaches of west coast permanently open estuaries is well documented and may partly explain why euryhaline zooplankton assemblages in these Cool Temperate estuaries are not unique. Euryhaline zooplankton assemblages from Cool Temperate and Warm Temperate south coast provinces do not represent different communities, with the transition towards a different biogeographical assemblage first appearing east of Port Elizabeth. This is in contrast to the nearshore cold water zooplankton assemblage, the more tolerant of which penetrate the colder lower reaches of the Great Berg estuary where salinity is still temporarily variable.

Key words:

• zooplankton
• Great Berg Estuary
• water temperatures
• biogeography.