IMPORTANCE OF RIVERINE OUTFLOW TO ECOSYSTEM FUNCTIONING IN THE KWAZULU-NATAL BIGHT, SOUTH AFRICA
Morag Ayers, Ursula Scharler and Sean Fennessy
University of KwaZulu-Natal, School of Life Sciences
Coastal marine ecosystems can be affected by anthropogenic activities in neighbouring systems, such as water abstraction and impoundments in riverine and estuarine ecosystems, and within the system itself, such as fishing. In estuaries, decreased river outflow can lead to inlet closures, while in oligotrophic coastal areas it can lead to decreasing nutrient inputs. In the oligotrophic KwaZulu-Natal Bight the inflow of numerous rivers and estuaries act as nutrient sources. In addition, penaeid prawns, targeted by trawlers in the central Bight, use two estuarine systems as nursery habitats. However the inlet of one of these, the St. Lucia Estuary, has been closed since 2002 due to decreased river flow. To identify the importance of river outflow to ecosystem functioning in the Bight, food web models representing carbon, nitrogen and phosphorus flows in the southern, central and northern Bight were constructed and analysed using ecological network analysis. To determine the effects of trawling and prawn nursery loss time-dynamic models of the central Bight were constructed and fitted to catch data for 1990 – 2009. The Thukela River flows into the central Bight and this area showed larger carbon, nitrogen and phosphorus biomass than southern and northern areas. System recycling differed between elements and seasons indicating that changes in the amount and elemental composition of river outflow could impact system functioning. The closure of St. Lucia positively affected benthic fish and carnivorous benthos biomass and negatively affected prawn, commercial crustacean and benthopelagic fish biomass. Negative effects of nursery loss were exacerbated and positive effects were decreased by high versus low trawling effort. Overall, results suggest decreases in riverine outflow could impact marine fisheries catches through decreases in nutrients, detritus and nursery habitat availability impacting ecosystem functioning.

 

Presentation topic:

IMPORTANCE OF RIVERINE OUTFLOW TO ECOSYSTEM FUNCTIONING IN THE KWAZULU-NATAL BIGHT, SOUTH AFRICA

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