Ecosystem development during open and closed phases of temporarily open/closed estuaries on the subtropical east coast of South Africa U.M. Scharler School of Biological and Conservation Sciences, University of KwaZulu-Natal, Westville Campus, Durban, South Africa article info Article history: Received 9 February 2011 Accepted 3 August 2011 Available online 10 August 2011 Keywords: ecosystem development ascendency temporarily open/closed estuaries biomass variability fitness ecosystem indices abstract Temporarily open/closed estuaries, or TOCEs, are the predominant type of estuary along the South African coast. Typically cut off from the sea during parts of the year, their ecosystems feature various developmental stages due to inlet breaching and the following closure. A full cycle ranges from breaching to drainage into the sea, thereafter a short period of tidal estuary followed by closure and a closed phase until breaching occurs again. Documented differences in biomass of various biota during open and closed phases of the Mdloti and Mhlanga estuaries over a seasonal cycle were translated into ecosystem networks. From these, ecosystem indices were calculated, and several (especially A/DC, AMI) reflected the changes between open and closed phases on the ecosystem level, and changes along a gradient of number of days the estuary has been open or closed. Differences in the developmental stages of the estuaries thus not only occurred at the community level, but also at the ecosystem level in terms of flow distribution and amount of carbon processed along the various trophic links. Furthermore, the fitness, or sustainability of system configuration, of the estuaries along the seasonal cycle was estimated and showed both estuaries to have a sustainable configuration, despite the periodicity of the open and closed inlet to which they are subjected, their altered freshwater inflow regimes, and some pollution. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Estuaries provide numerous ecosystem services including high biotic productivity, while they act as natural purifiers of inflowing river water, serve as nursery area for many fish and invertebrate species, provide food sources for human consumption, and are sites for recreation. In general, estuaries are high value ecosystems on the world’s coast. In South Africa, a relatively small proportion of estuaries are permanently open to the sea, whereas the dominant type of estuary (71%) is that of temporarily open/closed estuaries, or TOCEs (Whitfield, 2000), also named intermittently closed estuaries in other parts of the world. South Africa features 258 estuaries in total, with 93 TOCEs alone in the subtropical coastal region of the east coast of South Africa, from Mbashe (32  15 0 S 28  54 0 E) estuary in the south to Kosi Bay (26  54 0 S 32  52 0 E) in the north (Whitfield, 2000). Only about 10% of KwaZulu-Natal (KZN) estuaries are of the permanently or predominantly open type, whereas 73 estuaries are classified as TOCEs (Begg, 1978). Although TOCEs are generally small in size, they fulfil an important function in providing habitat for estuarine dependent species, and along the KZN coast that stretches for 590 km, these habitats occur at frequent intervals along with the few permanently open estuaries (POEs), two estuarine bays and two estuarine lakes in the far north. TOCEs provide not only a habitat for estuarine species, but fulfil the same important functions as the POEs or estuarine lake systems, however on a smaller scale in terms of productivity of the entire system. TOCEs on the KZN coast are predominantly closed during the dry winter season, but open frequently during the wet summer season. The timing of inlet opening is dependent on the amount of freshwater inflow, the inlet dimensions, and the width of the sandbar between the estuary and the sea. KZN TOCEs are in general perched, i.e. the water level during the closed phase rises well above mean sea level. Once it reaches the height of the sand berm separating it from the sea, seepage and erosion initiate mouth opening and the estuary empties into the sea. Thereafter follows a short period of tidal exchange, after which the estuary closes again (Whitfield, 1992). Closure is facilitated by sand transport of a long-shore drift in a south to north direction, and limited scouring through tidal action during the open phase. The importance of estuaries along the KZN coast can be sum- marised as follows. Firstly, they provide nursery areas for fish and invertebrate species (Perissinotto et al., 2010), a large proportion of which is of commercial importance in the in- and nearshore marine systems in terms of line- (Lamberth and Turpie, 2003; E-mail address: scharler@ukzn.ac.za. Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.08.003 Estuarine, Coastal and Shelf Science 108 (2012) 119e131