Dynamics of macrophytes in the East Kleinemonde, a small temporarily open/closed South Afrcan Estuary

The East Kleinemonde Estuary is one of 175 temporarily open/closed estuaries (TOCEs) that represent 70 percent of estuaries in South Africa. TOCEs are small (mostly less than 100 ha), shallow estuaries (average depth < 2 m) that respond quickly to freshwater inflow events. Their connection to the sea can be highly variable resulting in considerable changes in abiotic and biotic conditions. Mouth status depends on a balance between freshwater inflow and marine influence, which in turn affects ambient abiotic conditions. The objective of the study was to identify the abiotic variables which influence macrophyte growth and habitat availability. It was hypothesised that water level and salinity were the two main drivers of macrophyte change and macrophyte habitat would respond very rapidly, in less than a month, when habitat was available. Macrophyte habitats would also have high sediment seed reserves to ensure persistence under highly variable abiotic conditions. Macrophyte cover was monitored monthly in the East Kleinemonde Estuary along three permanent transects. The dominant habitats were submerged macrophytes, intertidal salt marsh, supratidal salt marsh, reeds and sedges. The following abiotic variables; water level, water column salinity, water temperature, Secchi depth, air temperature and rainfall were also measured between March 2006 and January 2010. Time-lag responses of the macrophytes to water level and salinity changes up to four months prior to the sampling session were also assessed. The analysis of a one year dataset highlighted only water level as a driver of change in macrophyte cover, whereas the five year dataset identified salinity as an additional important abiotic driver. This is because during September 2008 to January 2010 a series of large marine overwash events maintained high salinity (> 30 ppt) and high water level (> 1.6 m amsl) in the estuary. Water level increased by up to 0.33 m due to large volumetric changes and salinity was significantly higher in the 16 month closed euhaline phase after the breach (31 ± 0.9 ppt) compared to 21.9 ± 0.9 ppt in the closed polyhaline phase before the September 2008 breach. This increase in salinity significantly reduced the cover of the submerged macrophytes Ruppia cirrhosa and Chara vulgaris. They were replaced by macroalgae during this high salinity phase. The cover of supratidal salt marsh and reed habitats was also significantly reduced during the high water level phase, which in turn would lead to the potential for bank destabilisation and erosion. Based on the average elevation above sea level position of the macrophytes in the East Kleinemonde iv Estuary, a threshold water level was identified as 1.55 amsl. This was taken to be the height above sea level at which there was a maximum cover change for each macrophyte habitat. Above this water level emergent macrophyte habitat would mainly be inundated. This, together with 30 ppt salinity, was identified as the two thresholds for macrophyte change in the East Kleinemonde Estuary. From these thresholds and the 5 year dataset four biotic states were identified as State A: open and tidal, State B: closed with a water level below 1.55 m amsl and salinity between 18 to 30 ppt, State C: closed and water level above 1.55 m amsl and salinity between 18 to 30 ppt and State D: closed and water level above 1.55 m amsl and salinity above 30 ppt. Intertidal salt marsh, reeds and sedges were dominant during the open phase. Submerged macrophytes were dominant during the closed polyhaline state and macroalgae during the closed euhaline state. The high variability of abiotic factors common in TOCEs and the response of macrophyte habitat indicated that macrophytes were resilient to changing states provided they were of relatively short (< 3 months) duration. Macrophytes in the East Kleinemonde Estuary were found to have fast growth rates and large seed reserves in the sediment. The seed banks in the East Kleinemonde, as well as the adjacent temporarily open/closed West Kleinemonde Estuary were quantified for the first time in a South African estuary. The averaged data from both estuaries showed that Charophyte öospores represented almost 72 percent of the sexual propagules in the sediment with a mean öospore density of 31 306 ± 2 293 m-2. This was despite the Charophytes being sparsely located and only representing a maximum of 32.5 percent cover in the above ground vegetation. Historically there must have been stands of Charophytes in the East Kleinemonde Estuary, such that öospores could accumulate to such high density found in this study. The second highest seed density was for the intertidal salt marsh plant Sarcocornia tegetaria (18 percent) (7 929 ± 688 seed m-2), followed by the submerged angiosperm Ruppia cirrhosa (7 percent) (2 852 ± 327 seeds m-2). Although seed density did not differ significantly with sediment depth, seeds still occurred at 20 cm below the surface of the sediment providing a regeneration source in the event of sediment scouring during a flood event. Germination studies in the greenhouse showed that most seeds were viable and Sarcocornia tegetaria began to germinate after 3 days to a maximum of 82 percent after 91 days. Submerged species only germinated after 18 days with a low maximum germination of between 11 and 15 percent. This study has made an original contribution to the field of knowledge on macrophyte responses in a small TOCE as it showed that macrophyte habitats in the East Kleinemonde Estuary have a high natural variability in cover over time, they respond quickly after a disturbance event such as a mouth breach and there are large sediment seed reserves that remain viable from 2 to more than 5 years. This ensures habitat persistence even under unfavourable conditions, such as prolonged periods of mouth closure with high water level and flooding which causes loss of salt marsh species. Given this natural variability it is necessary to predict responses both spatially and temporally in order to manage and maintain ecological functioning in TOCEs. This study identified dominant macrophyte habitat for different abiotic states through the use of water level and salinity thresholds. In the determination of the freshwater requirements of any South African estuary freshwater inflow rates are provided for each estuary's past, present and possible future freshwater inflow scenarios. These flow data are generated by hydrological models and simulated monthly inflow volumes for a period of about 72 years are provided. For the East Kleinemonde freshwater requirement study for any year in that 70-odd year period, the number of high flow and low flow mouth breaches were predicted, as well as the closed state periods. The threshold water level of 1.55 m amsl was also used to filter past, present and future inflow monthly volumes to determine the frequency of the four abiotic states identified in this study. It was based on a water level/water volume equation calculation from a digital elevation model. Results showed that the total closed period in the present state was 83 percent, made up of 48 percent of the time in a polyhaline state (State C) and 35 percent in a euahaline state (State D). A second method was used to quantify available spatial habitat under different water level scenarios. A spatial model was written in Model Builder, an application in ArcGIS that allowed a series of processes to be built. A habitat map was overlaid with a bathymetric map and by selecting water level, available habitat areas were determined and empirical equations of water level versus available habitat were produced. These equations were then used to calculate the available habitat areas for monthly water level conditions from the freshwater requirement study for the past, present and two future inflow scenarios. Using both the threshold water level method and the spatial availability model method it was possible to assess the effect of the two future inflow scenarios on macrophyte habitat vi response. Scenario 1 had a 16 percent reduction in mean annual runoff (MAR) generating low flows for 88.6 percent of the time and a 3.5 percent reduction in flood events. In Scenario 2 there would be a 12 percent reduction in MAR with low flows occurring for 87.5 percent of the year, a 5.3 percent reduction in floods and an 11.5 percent reduction in the open mouth state. The model showed that Scenario 1 would have the highest submerged macrophyte area (12.56 ha versus 12.48 ha in Scenario 2), whereas Scenario 2 produced the largest mudflat and intertidal salt marsh area (7.11 ha versus 7.34 ha) due to lower water level in conjunction with the bathymetry of the estuary. A reduction in freshwater inflow to TOCEs either due to anthropogenic influences or natural precipitation cycles is one of the main threats to the optimum functioning of these estuaries. The results from this study and the two methods of assessing the effect of freshwater inflow scenarios on macrophytes in TOCEs can be integrated into the current freshwater inflow assessment methodology in South Africa, as well as adding to our understanding of the ecological functioning of these small, highly variable estuaries. The methods provide a quick assessment of macrophyte habitat associated with abiotic states under past, present and future inflow scenarios. All that is required to predict macrophyte habitat for different freshwater inflow scenarios (present, past and future) is a habitat map, a bathymetric map and the elevation range of macrophytes in the TOCE being assessed. This, together with the knowledge of response rates, provides invaluable information for the management of TOCEs to maintain their ecological functioning under altered freshwater inflow regime