A high proportion of South Africa’s temporarily open/closed estuaries (TOCEs) occur along
the coast of KwaZulu-Natal. Mouth breaching events have major impacts on the biological
processes of an estuary, resulting in depletion of zooplankton via flushing and sediment
scouring. Mouth closure, usually within weeks of a breaching event, initiates a new phase of
stable physical conditions, leading to biological recovery. Therefore, the aim of this
investigation was: (1) to monitor the recovery of zooplankton abundance and biomass
following a breaching event in the Mdloti Estuary; (2) to compare the spatial and temporal
patterns in zooplankton distribution in the lower (mouth), middle and upper reaches (head) of
the Mdloti Estuary in terms of abundance and biomass just before, during and after a mouth
breaching event; and (3) to determine the key environmental variables influencing
zooplankton abundance and biomass during such a breaching event. The zooplankton
community of the Mdloti Estuary was studied over a 3-month period (27 January to 26 April
2004). The estuary was artificially breached on 12 February 2004, due to a fish kill, and
closed again naturally on 18 March 2004. Samples were collected twice a week in the lower,
middle and upper reaches using a WP-2 net and an epibenthic sled. Upon breaching, 98% of
zooplankton biomass was lost through sediment scouring and flushing. During the open
phase, zooplankton biomass showed a temporary recovery, but due to continual sediment
scouring and flushing, this was not sustained. One-way ANOVA revealed a significant
difference in total zooplankton abundance and biomass between phases (d.f.2, 59 = 55.0; p <
0.001; d.f.2, 59 = 15.51; p < 0.001). ANCOVA revealed significant differences between
zooplankton abundance and biomass (d.f.0.05;2,56=2.97, p = 0.05) at the different estuarine
reaches (d.f.0.05;2,56=5.51, p < 0.01) . In both cases, the lower reaches recovered quicker than
the middle and upper reaches. Thirty-five taxa were identified during the study, with only 10
contributing more than 1% of the total abundance or biomass. For the overall study, P. hessei
was the dominant species, accounting for 42% of the total abundance and 58% of the total
biomass. Keratella sp. 1 accounted for 17% and 11% of the total abundance and total
biomass, respectively, while harpacticoid copepodites and Acartia natalensis contributed 11%
and 10% to the total zooplankton abundance and 3% and 7% to the total zooplankton
biomass, respectively. Pre-breaching levels of zooplankton were reached only 9 days after
the closure of the mouth, during the recovery phase (mean 1.1 x 105 ± 6.5 x 104 SD ind.m3
and 2.4 x 102 ± 1.6 x 102 SD mg.m3). Zooplankton abundance and biomass reached a peak in
the lower reaches after 19 days, in the upper reaches after 28 days and in the middle reaches
after 35 days. The zooplankton biomass decreased slightly, but stabilised for the duration of
the study. During the study the state of the mouth was primarily responsible for regulating the
zooplankton abundance and biomass. However, the zooplankton in the different reaches did
not recover in synchrony after mouth re-closure because abiotic factors and food availability
were different in the three estuarine reaches. / Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2010.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/8419 |
| Date | January 2010 |
| Creators | Deale, Maria. |
| Contributors | Perissinotto, Renzo. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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