P. W. FRONEMAN, P. D. VORWERK
412
worldwide, have been reported to be related to the inter-
active effects of temperature and food availability [2,5,17]
The influence of temperature can largely be discounted
as water temperatures were broadly similar during the
two seasons The significant increase in zooplankton den-
sity and biomass recorded from dry to wet season during
this study is, therefore, likely to be the result of elevated
food availability. While the dry season biomass and den-
sities values are in the range reported for Kariega Estuary,
the wet season densities and biomass are substantially
higher and are in the range reported for permanently
open southern African estuaries with sustained freshwa-
ter inflow [2-4,13]. Results of the hierarchical cluster
analyses indicated that the wet and dry seasons were cha-
racterised by distinct zooplankton communities (Figure
7). The species which demonstrated the greatest increase
in numbers between dry and wet season was the copepod,
Acartia longipatella, which contributed ≈8.5% of the
total abundance during the dry season, but represented
≈75% of the total abundance during the wet season. Al-
though the actual abundances of Pseudodiaptomus hessei
increased from dry to wet season, the percentage contri-
bution of the total abundance decreased from ≈25% to
15%. Successional patterns of copepods within southern
African estuaries are largely driven by alterations in sa-
linity [2]. Acartia longipatella reportedly attains the high-
est abundances and biomass during periods when oligo-
haline conditions prevail [2,5,21]. Conversely, the calan-
oid copepod, P. hessei can be considered as a pioneer
species able to tolerate a high variance in salinity and
temperature [2,21,22]. The observed shift in the numeri-
cally dominant copepod species from the dry to wet phase
can therefore be attributed to a change in the salinity re-
gime within the estuary resulting from the freshwater
pulse.
Results of the numerical analyses conducted during the
dry and wet season indicated the presence of a longitu-
dinal gradient in the zooplankton assemblages within the
Kariega estuary. During the dry season, those stations oc-
cupied in the lower reaches of the estuary were distinct
from the stations within the middle and upper reaches of
the system. The clear separation of the two groupings
could largely be attributed to the increased contribution
of marine species (copepods of the genera Oithona, Eu-
calanus and Calanus) to the total counts at stations in the
lower reaches reflecting the influence of the marine en-
vironment on the estuary. On the other hand, during the
wet season, the upper reach stations separated from those
occupied within the middle and lower reaches of the es-
tuary. SIMPER analyses indicated that the separation
could be largely ascribed to reductions in the numerical
abundances of the dominant copepods within the upper
reaches of the estuary. The reduced abundances within
the upper reaches can probably be ascribed to the inflow
of freshwater which would prevent the build up of zoo-
plankton biomass within the region.
Results of this study indicate that the freshwater pulse
into the Kariega Estuary was associated with an increase
in the zooplankton biomass and a shift in the zooplankton
species composition. The horizontal patterns in zooplank-
ton community structure and biomass can be ascribed hy-
drodynamics of the estuary, reflecting both the magni-
tude of freshwater inflow into the system and the influ-
ence of the marine environment on the lower reaches of
the estuary. The increase in the phytoplankton biomass
associated with the freshwater inflow is also likely to be
associated with a change in the food web structure from a
detrital food web to one where the classical food web
predominates [23]. Additionally, the outflow of estuarine
water is also likely to be associated with elevated pri-
mary and secondary production rates in the near shore
marine environment [24].
6. Acknowledgements
The authors would like to thank the National Research
Foundation (NRF) of South Africa and the South African
Observation Network (SAEON) Elwandle Node for pro-
viding funds and facilities tocomplete this study.
REFERENCES
[1] J. Adams, G. Bate and M. O’Callagan, “Estuarine Micro-
algae,” In: B. R. Allanson and D. Baird, Eds., Estuaries of
South Africa, Cambridge University Press, Cambridge,
1999, pp. 91-100.
[2] T. H. Wooldridge, “Estuarine Zooplankton Community
Structure and Dynamics,” In: B. R. Allanson and D. Baird,
Eds., Estuaries of South Africa, Cambridge University
Press, Cambridge, 1999, pp. 91-100.
[3] B. R. Allanson and G. H. L. Read, “Further Comment on
the Response of Eastern Cape Province Estuaries to Vari-
able Freshwater Inflows,” South African Journal of Aqua-
tic Science, Vol. 21, 1995, pp. 56-70.
[4] N. Grange and B. R. Allanson, “The Influence of Fresh-
water Inflow on the Nature, Amount and Distribution of
Seston in Estuaries of the Eastern Cape, South Africa,”
Estuarine, Coastal and Shelf Science, Vol. 40, No. 4,
1995, pp. 403-420. doi:10.1006/ecss.1995.0028
[5] H. L. Jerling and T. H. Wooldridge, “Plankton Distribu-
tion and Abundance in the Sundays River Estuary, South
Africa, with Comments on Potential Feeding Interac-
tions,” South African Journal of Marine Science, Vol. 15,
1995, pp. 169-184. doi:10.2989/02577619509504842
[6] P. D. Vorwerk, “A Preliminary Examination of Selected
Biological Links between four Eastern Cape Estuaries
and the Inshore Marine Environment,” Ph.D. Thesis,
Rhodes University, Grahamstown, 2006, pp. 1-268.
[7] G. C. Bate, A. K. Whitfield, J. B. Adams, P. Huizinger
and T. H. Wooldridge, “The Importance of the River-Es-
tuary Interface (REI) Zone in Estuaries,” Water SA, Vol.
Copyright © 2013 SciRes. JWARP