Phytoplankton/protozoan dynamics in the Nyara Estuary, a small temporarily open system in the Eastern Cape (South Africa)

Abstract

Most estuarine studies in South Africa have focused on larger estuaries, while numerous, smaller, temporarily open estuaries have received little research attention. This study examines the phytoplankton distribution in relation to physical and chemical variables through a quasi-seasonal cycle (from March to November 1997, including a flood event) in the Nyara, one of the smaller Eastern Cape estuaries. Vertical stratification of salinity was a persistent feature, indicating limited mixing of the water-column, except during flood events. Average chlorophyll-a concentrations for the estuary decreased from 1.3μg/l in March to 0.1μg/l in September, corresponding to the seasonal decline in rainfall and freshwater inflow. With the exception of the period during the November flood, the shallowness of the estuary as well as low chlorophyll-a concentrations always allowed an average of 30% of surface light to reach the sediment surface. Diatoms were most abundant in the water-column in March (mean = 105 cells/l), but numbers declined to below 104 cells/l in May and September. Picoplankton numbers increased over the study period, while nanociliates and dinoflagellates were fairly constant at 104 and 105 cells/l respectively. The most common group in terms of carbon, were the dinoflagellates. Freshwater, or possibly the input of nutrients associated with it, appears to be the most significant factor affecting phytoplankton biomass in the Nyara Estuary. When not receiving pulses of nutrients through freshwater inflow, the Nyara is best described as a predominantly low nutrient, low phytoplankton biomass, stratified system, dominated by the microbial food-web and possibly fed by detritus. This state can however, be altered rapidly by flood events that cause mouth opening.


Keywords: estuary; temporarily open; phytoplankton; protozoans; clorophyll-a; carbon; biomass


(Afr J Aqua Sci: 2001 26(1): 31-38)