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.
Male and female genital morphology of the spirostreptid millipede Orthoporus pyrhocephalus was examined by conventional and scanning electron microscopy techniques. The relative syncopulatory positions of the genitalia were established through dissections of pairs in copula. Male gonopods bear scoops and regions of pitted spines. both of which are similar in form to morphological structures implicated in insect sperm displacement. Consideration of the position of the oviduct‐spermathecal junction in females predicts that displacement is restricted to a sperm removal mechanism. The importance of spermathecal shape in determining sperm precedence patterns is discussed. The orientation of the genitalia during copulation indicates that sperm displacement is plausible and a mechanism of gonopod action is suggested.
The relationships between the standing stocks of deposit-feeding benthic invertebrates and benthic chlorophyll-a, phaeopigment and total combustible organic matter were investigated at a series of coastal lagoons and in the type of intertidal soft-sediment sites from which the lagoons originated. Across all the sites, in Norfolk, UK, an inverse relationship occurred between (a) the amounts of chlorophyll-a and of other potential food materials and (b) the degree of coverage by water. The biomass of consumers also decreased with increased water coverage, so that the lagoons supported less biomass than the adjacent high-level intertidal sites. Further, the deposit-feeder biomass supported by unit food decreased with extent of water coverage.There was no evidence of any relationship between deposit-feeder and food biomass within any single site, in spite of the study period being selected to be that in which there was maximum likelihood of competition for microphytobenthic food. Whilst chlorophyll concentrations may set the maximum achievable level of consumer biomass at these sites, including in the deeper lagoons setting very low potential maximum population densities, the seasonal abundance patterns of the deposit feeders appear to be determined by other factor(s).
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