The response of two South African east coast estuaries to altered river flow regimes

Abstract

1. Major reductions in catchment run-off, a result of frequent and prolonged drought conditions, together with extensive impoundment of rivers and streams, has led to concern about the possible negative effects on downstream estuaries. Preliminary studies have shown that changes in river flow and associated nutrient inputs have had a predominantly negative impact on the aquatic biota of Eastern Cape estuaries. Natural successions now have human imposed trajectories which are reducing variability and forcing both freshwater 'deprived' and freshwater 'enriched' systems into artificial cycles.2. The Kariega and Great Fish estuaries in the Eastern Cape Province are used in a comparative manner to illustrate how differences in riverine inflow can influence the structure and functioning of selected biotic components in permanently open systems. Maximum chlorophyll a values in the freshwater deprived Kariega Estuary were 1 mg L − 1 , whereas maximum values in the freshwater enriched Great Fish Estuary were 22 mg L − 1 . Mean zooplankton biomass in the lower, middle and upper reaches of the Kariega Estuary was always below 50 mg m − 3 , whereas in the same reaches of the Great Fish Estuary, these values ranged from 256 to 4253 mg m − 3 . Similarly, mean ichthyonekton densities in the mouth region of the Kariega Estuary were 49 individuals per 100 m 2 compared with 279 per 100 m 2 in the Great Fish Estuary.3. Temporal changes of riverine flow reinforce the importance of allochthonous inputs to the functioning of Eastern Cape estuaries. A comparison between a dry and wet period in the Kariega Estuary revealed mean nitrate and phosphate concentrations increasing from 5 to 101 mmol L − 1 and from 1 to 5 mmol L − 1 , respectively. Phytoplankton stocks responded positively to freshettes in both the Kariega and Great Fish estuaries. Similarly, peaks in zooplankton biomass in the Great Fish Estuary increased rapidly in response to high food resource availability resulting from elevated river discharge. The higher ichthyoplankton and ichthyonekton densities in the Great Fish Estuary, when compared with the Kariega Estuary, were attributed to a combination of stronger olfactory cues for larval immigrants from the sea and elevated food stocks in the former system.

Response of coastal marine eco-environment to river fluxes into the sea: A case study of the Huanghe (Yellow) River mouth and adjacent waters

“…The changes in these rivers have significantly affected hydrographic conditions, salinity and temperature patterns, nutrient loads, dissolved oxygen (Skliris and Lascaratos, 2004), and, in turn, the primary productivity, biotic community structure and distribution, and the trophodynamics of estuarine systems (Grange and Allanson, 1995;Grange et al, 2000;Howden and Murtugudde, 2001;Kennish, 2002;Kimmel and Roman, 2004;Kimmerer, 2002). It is generally acknowledged that these kinds of changes have brought about detrimental outcomes, such as eutrophication, sediment deprivation, hypoxia, and contamination, as well as habitat loss and decreased bio-diversity, in many estuarine and coastal areas worldwide (Grange et al, 2000;Kennish, 3 2002; Rodriguez et al, 2001;Sklar and Browder, 1998;Wilber and Bass, 1998). Very few estuarine and coastal systems remain intact in the world.…”

Response of the Plankton to a Fresh Water Pulse in a Fresh Water Deprived, Permanently Open South African Estuary

“…Despite the permanently open nature of the Kariega Estuary, the system is currently highly impacted due to fresh water deprivation resulting from impoundments along the Kariega River [4,17]. The recorded normal [11,13,21].…”

“…The increase in the total chl-a concentration in freshwater dominated estuaries is largely thought to be result of elevated phytoplankton production rates resulting from increased macronutrient availability [1,3,17]. Alternatively, it is also possible that the elevated total chlorophyll-a concentrations recorded during the wet season may have be derived from riverine input or the resuspension of microphytobenthic algae [4,13,17].…”

“…Shifts in the total zooplankton abundances and biomass within southern African estuaries and indeed estuaries 412 worldwide, have been reported to be related to the interactive 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 density 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 densities 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 freshwater inflow [2][3][4]13].…”

“…The dominance of the marine environment within the system is indicated by the 106:1 ratio of tidal prism volume to river flow volume [17]. The middle reaches of the estuary are characterised by extensive salt marshes comprising the high marsh plants Sarcocornia perennis, Chenolea diffusa and Spartina maritima.…”