Invertebrate plankton assemblages in the surf zones of St. Lucia and Mhlathuze Estuaries, KwaZulu-Natal, South Africa
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Date
2002
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Abstract
This study described the surf zone invertebrate plankton assemblages of the exposed sandy beaches adjacent to the St Lucia and the Mhlathuze estuaries. The broad aims of this study were to identify the invertebrate plankton assemblages, to describe these in terms of species composition, abundance and spatio-temporal changes, and to relate these communities to adjacent estuarine fauna. Physico-chemical variables which influenced the community structure were investigated. Samples previously collected at St Lucia between February 1992 and January 1993 by Harris (1996) were used. Additional sampling was carried out monthly from November 1998 until October 1999 in the Mhlathuze surf zone during the day and night Invertebrate assemblages in the surf zones described in this study were compared with their adjacent estuaries to investigate the relationship between these habitats..
tn total, 171 taxa were collected from both systems. Total abundances ranged from Q.31 to 1570 and 0.17 to 106 individuals/m3 at the St Lucia and Mhlathuze surf zones respectively. Plankton and bentho-planktonic organisms belonging to different taxonomic types formed the major component of the assemblages in the St Lucia and Mhlathuze surf zones. The surf zone invertebrate communities were mainly dominated by crustaceans, with the mysids Gastrosaccus spp. and brachyurans (megalopae) dominating the taxa at St Lucia and the Mhlathuze estuaries respectively. Collectively copepods dominated the boioplanktonic forms. Other commonly abundant hotoplankton included siphonophores, chaetognaths, dadocerans and medusae. The meroplankton composed of polychaetes and decapod larvae which dominated this group. Decapod larvae included pagurid, brachyuran and shrimp-like forms. Benthoplanktonic crustaceans sampled from these surf zones included surf zone/beach mysids of the genus Gastrosaccus, the isopod Grolana spp. and amphipods.
Classification and ordination analyses revealed distinct day and night communities, with less seasonal and spatial (site) differences. Nine and eight fauna! assemblages separated on basis of season were identified at St Lucia and the Mhlathuze respectively. At St. Lucia four day assemblages were identified: (i) the day summer/autumn assemblage characterised by the dominance of calanoids, brachyuran megalopae and zoeae, penaekt prawns, the copepod Corycaeus sp.; (it) the day summer assemblage characterised calanoids, Sagitta sp., Lucifer sp., brachyuran megalopae and Gastrosaccus spp.; (iii) the day autumn/winter characterised by Sagitta sp., calanoids, and Oro/ana spp.; (iv) the day spring assemblage characterised calanoids, Sagitta sp., Gastrosaccus spp. brachyuran megalopae, Lucifer sp. and penaeid prawns. Within the night community five assemblages were distinguished: (i) the night summer assemblage characterised by Gastrosaccus spp., Grolana spp., brachyuran megalopae and zoeae, calanoids and cumaceans; (ii) the night autumn assemblage dominated by Gastrosaccus spp., calanoids, Grolana spp., brachyuran megalopae, and Sagitta sp.; (iii) the night autumn/winter assemblage characterised by the dominance of Grolana spp., calanoids, Gastrosaccus spp. and brachyuran megalopae; (iv) the night spring assemblage characterised by Gastrosaccus spp., calanoids, and Grolana spp.; (v) the night spring/autumn assemblage characterised by the dominance of calanoids, Grolana spp., and brachyuran megalopae.
In the Mhlathuze surf zone, the day community was formed by four assemblages which included: (i) the Mhlathuze day summer/autumn assemblage dominated by brachyuran megalopae and zoeae, ctenophores, and calanoids; 00 me Mhlathuze day winter assemblage characterised by the dominance of calanoids, ctenophores and brachyuran megalopae; (iii*) the Mhlathuze day winter/spring assemblage characterised by calanoids, siphonophores and Sagitta sp.; (tv) the Mhlathuze day spring/summer characterised by brachyuran megalopae, Grolana spp. and Gastrosaccus spp. Within the night community at the Mhlathuze, four assemblages were identified which included: (i) the Mhlathuze night summer assemblage characterised by the dominance of brachyuran megalopae, Grolana spp., and calanoids; (ii) the Mhlathuze night autumn assemblage characterised by the dominance of Grolana spp., brachyuran megalopae, Gastrosaccus spp. and ctenophores; (iii) the Mhlathuze night spring/summer assemblage characterised by brachyuran megalopae, Grolana spp. and calanoids; (iv) the Mhlathuze night winter/spring assemblage characterised by the dominance of calanoids, brachyuran megalopae, Gastrosaccus spp. and Grolana spp. Assemblages in these surf zones showed diverse invertebrate plankton taxa which originated from different habitats such as intertidai and offshore marine environments and nearby estuaries.
The significant differences recorded between day and night communities were attributable to the nocturnal activity of many crustacean bentho-planktonic groups including beach/surf zone mysids, isopods and decapod larvae in the St Lucia and Mhlathuze surf zones. At St Lucia, significant differences were found between summer and winter communities, while at the Mhlathuze significant differences were recorded between seasonal groups. With respect to site, there were no significant differences recorded at St Lucia and Mhlathuze. This indicated that there was no apparent accumulation of planktonic invertebrates in the surf zone near the estuary mouths.
Significant differences were also recorded between the St Lucia and Mhlathuze surf zones in terms of their community structure, abundance and diversity where higher densities were recorded at the St Lucia estuary. These results reflected differences in physical processes and conditions that the St Lucia and Mhlathuze surf zone communities were exposed to. These included currents, the extent of freshwater input from the estuary, wind-dri ven-wa ve action, temperature, salinity and turbidity. Significant differences between the St Lucia and Mhlathuze communities were also ascribed to a dry period (1992/93) at St Lucia compared with the Mhlathuze which was sampled during a wet period (1998/99). However, salinity and temperature were the most influential physico-chemical factors structuring the two communities. Although variables such as salinity and longshore currents may have influenced the surf zone communities, distinct temperature-related, seasonal patterns occurred with decreased densities in winter.
A comparison of surf zone plankton recorded in this study with estuarine communities indicated the existence of a faunal relationship between the surf zones and estuaries of the St Lucia and Mhlathuze systems. Densities of common taxa were slightly higher in the surf zone than in the estuaries, emphasising the importance of these surf zones as habitats rather than being used as a transient area between the estuary and offshore environments. The St Lucia and Mhlathuze surf zones were important to many faunal groups which included the mysid Gastrosaccus spp., brachyurarts (megalopae and zoeae), the isopod Orolana, copepods and chaetognaths. However, during present study it was difficult to demonstrate the utilization of these surf zones as nursery grounds due to the use of 500 mesh which precluded the collection some larval stages. It is therefore recommended that more detailed research be conducted with smaller mesh nets such as 60 ^im in these surf zones to better understand the importance of surf zone ecosystems.
Description
Submitted to the Faculty of Science in fulfilment of the requirement for the degree of Master of Science in the Department of Zoology at the University of Zululand, South Africa, 2002.
Keywords
Plankton--South Africa--St. Lucia estuary, Zooplankton--South Africa--St. Lucia estuary, Plankton--South Africa--Mhlathuze estuary, Zooplankton--South Africa--Mhlathuze estuary