Thomas G. Bornman

A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system

In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15–25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure (5–15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.

A Coastal Seawater Temperature Dataset for Biogeographical Studies: Large Biases between In Situ and Remotely-Sensed Data Sets around the Coast of South Africa

Gridded SST products developed particularly for offshore regions are increasingly being applied close to the coast for biogeographical applications. The purpose of this paper is to demonstrate the dangers of doing so through a comparison of reprocessed MODIS Terra and Pathfinder v5.2 SSTs, both at 4 km resolution, with instrumental in situ temperatures taken within 400 m from the coast. We report large biases of up to +6°C in places between satellite-derived and in situ climatological temperatures for 87 sites spanning the entire ca. 2 700 km of the South African coastline. Although biases are predominantly warm (i.e. the satellite SSTs being higher), smaller or even cold biases also appear in places, especially along the southern and western coasts of the country. We also demonstrate the presence of gradients in temperature biases along shore-normal transects — generally SSTs extracted close to the shore demonstrate a smaller bias with respect to the in situ temperatures. Contributing towards the magnitude of the biases are factors such as SST data source, proximity to the shore, the presence/absence of upwelling cells or coastal embayments. Despite the generally large biases, from a biogeographical perspective, species distribution retains a correlative relationship with underlying spatial patterns in SST, but in order to arrive at a causal understanding of the determinants of biogeographical patterns we suggest that in shallow, inshore marine habitats, temperature is best measured directly.

Environmental influences on living marine stromatolites: insights from benthic microalgal communities

Extant marine stromatolites act as partial analogues of their Achaean counterparts, but are rare due to depleted ocean calcium carbonate levels and suppression by eukaryotic organisms. Unique, peritidal tufa stromatolites at the interface between marine and freshwater inputs were discovered in South Africa in the past decade. Our aim was to investigate the benthic microalgal community (green algae, diatoms and cyanobacteria) of these stromatolites to assess succession and dominance patterns using real-time, in situ measurements of algal concentrations and composition. These biological measurements were modelled using generalized linear modelling (GLM) multivariate statistics against water physical and chemical parameters measured at regular monthly intervals, from January to December 2014. Salinity peaked and temperature dipped in winter, with both correlated to microalgal community change (GLM: P < 0.01). Diatoms and cyanobacteria, which construct the stromatolites, were consistently the dominant groups within the algal community, with minimal green algae present throughout the year. Importantly, this demonstrates a unique, relatively stable microalgal stromatolite community as opposed to those of other marine stromatolites, which likely require seasonal and stochastic disturbance to persist. This has implications in terms of interpreting community succession and differential layering in modern and fossilized stromatolites respectively.

Phytoplankton community dynamics within peritidal pools associated with living stromatolites at the freshwater–marine interface

Recently-discovered peritidal stromatolite ecosystems in South Africa form at the interface of freshwater seeps and the ocean intertidal zone, sharing several similarities with both tidal pool and estuarine ecosystems. While the overall ecology of tidal rock pools has been well studied, the dynamics of the phytoplankton assemblage have been comparatively neglected. In addition, there are no studies to date which describe the dynamics of phytoplankton within a habitat associated with stromatolites. The aim of this study was to investigate the coarse-scale phytoplankton community composition of a series of peritidal pools associated with living stromatolites, using a spectral fluorescence analysis tool, in relation to source-specific drivers related to both freshwater and marine forces. Three sites were sampled monthly from January to December 2014. Physico-chemical, biotic and meteorological parameters were recorded to assess some of the factors which might influence the phytoplankton size-fractionation and community composition using a generalised linear modelling approach. Results indicate that fresh or marine pool state, temporal differences associated with season, macronutrients (N and P), and benthic microalgal biomass are important drivers of the phytoplankton assemblages. Specifically, a transition from fresh to marine pool conditions resulted in an increased abundance of smaller phytoplankton size fractions and a shift from Chlorophyta and Cyanophyta to Bacillariophyta and Cryptophyta. Overall, the community was dominated by Chlorophyta and Bacillariophyta. There was consistency between the drivers and composition of the phytoplankton community compared to those from the few other comparable published studies. Furthermore, this study demonstrates a system which is dominated by benthic rather than pelagic microalgae in terms of biomass, thereby supporting the persistence of actively accreting stromatolites.

Towards a multigene phylogeny of the Cymatosiraceae (Bacillariophyta, Mediophyceae) I: novel taxa within the subfamily cymatosiroideae based on molecular and morphological data.

The family Cymatosiraceae, composed of two subfamilies (Cymatosiroideae and Extubocelluloideae), are elongate, non‐pennate diatoms and are commonly reported in marine planktonic and benthic assemblages all over the world. The combination of the gross bilateral symmetry of the frustule and radial symmetry of the valve poration, distinct pore fields at the valve apices and DNA sequence data place this family in the controversial diatom class Mediophyceae, sister to the pennate diatoms. The relationships of the Cymatosiraceae within the Mediophyceae, and the monophyly of the two cymatosiroid subfamilies, have become less stable with the addition of new DNA sequence data. In this paper, we examined 24 cymatosiraceaen strains, including the new taxa Lambertocellus africana (Dąbek & Witkowski) Dąbek, Witkowski & Ashworth comb. nov., Leyanella probus Ashworth, Dąbek & Witkowski sp. nov., and Leyanella pauciporis Ashworth, Dąbek & Park sp. nov. to create the most complete molecular phylogeny of the Cymatosiraceae to date, using a three‐gene (SSU, rbcL and psbC) data set. The results of the phylogenetic analysis supported the monophyly of the Cymatosiraceae, but not the monophyly of the subfamilies Cymatosiroideae and Extubocelluloideae. The phylogenetic analysis also suggested a close relationship of the Cymatosiraceae to the family Eupodiscaceae. The phylogenetic results have lead us to re‐evaluate the taxonomy of L. africana, which is sister to Cymatosira lorenziana rather than Minutocellus as suggested in the original description.