RESUMEN

This study forms the first basic assessment of microphytobenthos (MPB) dynamics in micro-estuaries and micro-outlets in southern Africa. It examines MPB community responses to environmental variables and further investigates MPB composition qualitatively across different micro-estuaries and micro-outlets over four seasons in a warm temperate region of the subcontinent. Combinations of multivariate analyses were used to explore similarities and differences in MPB communities between systems. Human-induced catchment changes between microsystems ranged from no alteration (rating 0; mostly micro-outlets) to extreme modification (rating 5; mostly micro-estuaries). Two hundred and sixty-seven MPB taxa were identified within all the microsystems, with 247 and 230 MPB taxa being observed in the micro-estuaries and micro-outlets, respectively. The MPB communities differed slightly in terms of microsystem types and seasons, but no significant differences were observed. Multivariate analyses (i.e. Boosted Regression Trees, Canonical Correspondence Analysis) showed that water column variables were significant and important in structuring MPB communities, with soluble reactive phosphorus, sediment pH, turbidity, ammonium and temperature being documented as key drivers. The MPB community composition clearly reflected the influence of catchment anthropogenic activities on species composition and structure. Moderately modified catchments resulted in MPB community structure variation among water bodies in relationship to land use and salinity gradients. The study found that; (i) by virtue of their size, microsystems and their catchments are likely to be particularly vulnerable to anthropogenic pressures when compared to systems of larger size; (ii) a typical impacted state may reflect reduced environmental heterogeneity which, compared to larger systems, may be achieved over much shorter time periods (following a particular event) or under much less intensive impacts; and (iii) the response in terms of MPB structure may predictably reflect a concomitant change from a complex community dynamic (structure and spatio-temporal attributes) to one that approaches a homogeneous structure (poor spatial zonation, strong taxonomic dominance, low species diversity).

Asunto(s)

Ecosistema , Estuarios , África Austral , Fósforo , Salinidad , Estaciones del Año

RESUMEN

This paper presents an environmental flow methodology that was developed to accommodate shallow, highly dynamic micro-tidal estuaries found along the wave-dominated coast of South Arica. This method differs to most other approaches that primarily focus on larger permanently open systems having unrestricted inlets. Following an adaptive, design science research approach, the 7-step method adopted both ecohydrological and ecosystem-based concepts, encapsulating key hydrologicalhydrodynamic-biogeochemical processes, as well as biotic responses. The procedure also addresses a key challenge often encountered in applying these approaches to complex estuarine systems - the mismatch of temporal and spatial scales between abiotic processes and biotic responses. The method simplifies and aggregates abiotic processes to appropriate scales suitable for analysis of biotic responses, by introducing concepts such zoning and major physical states that characterize an estuary. The method's flexibility in data requirements lends itself to applications in countries where data is limited or where differences exist in data quality between systems. Essential in any environmental flow determination process, however, is long-term monitoring to incrementally improve confidence of the input data, but also to evaluate whether allocated flows achieve desired objectives set. Future challenges include refining the method to accommodate flow changes within much shorter timeframes and in conjunction with escalating global change pressures amongst other; pollution, living resource exploitation and physical destruction of habitat.

RESUMEN

Ratios of 13C/12C and 15N/14N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in 13C with average δ13C values (-9.5±0.5‰) being typical of limnetic waters. Benthic detrital particles (δ13C-24.1±0.3‰) originated mainly from marginal vegetation (δ13C-25.7±0.3‰), but their slightly elevated carbon ratio suggests additional input from 13C-rich sources-possibly C4 plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (δ13C -32.5±0.3‰) foraged by filter-feeding on fine suspended particulate organic matter (δ13C-31.2±0.5‰). Amphipods (δ13C-28.0±0.6‰) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (δ13C-23.8±1.5‰) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.