Mesozooplankton community responses to a large-scale harmful algal bloom induced by the non-indigenous dinoflagellate Lingulodinium polyedra.

Our understanding of how zooplankton community composition varies in relation to harmful algal blooms remains limited, particularly in ecosystems where toxin-producing algae may have been introduced through anthropogenic activities. Harmful algal blooms (HABs) naturally occur on the coast of southern Africa, where they are predominantly associated with the cold Benguela region. In the warm-temperate waters east of Cape Agulhas, HABs occur rarely and red tides are mostly associated with the non-toxin producing dinoflagellate Noctiluca scintillans. Blooms of N. scintillans may cause water discolouration, but this is generally short-lived with limited impact on the ecosystem. However, in December 2013 the eastern Agulhas region experienced an extensive HAB, which persisted for ca. 4 months and affected >500 km of coastline, from Wilderness to East London. This unprecedented event was caused by the non-indigenous toxin-producing dinoflagellate, Lingulodinium polyedra. The impact on the coastal seas was widespread and severe, with instances of low dissolved O2 levels and fish kills being reported at the time in the broader Algoa Bay area. This study investigated the impact of the L. polyedra bloom on the mesozooplankton of Algoa Bay and reports the successive changes in zooplankton community composition and biomass observed from July 2013 to July 2014. The bloom impacted species diversity and richness, with a marked shift in dominance from a calanoid copepod dominated community to one dominated by microzooplankton (specifically cyclopoid copepods, tintinnids and cladocerans), over the period November 2013-March 2014. Calanoid copepod abundance was significantly reduced throughout Algoa Bay with the progression of the bloom, and this significantly impacted the total zooplankton biomass of the region. The results of the study suggest that harmful algal blooms have a negative impact on zooplankton communities, with notable implications for the higher trophic levels of the coastal pelagic ecosystem.

Micro-estuaries and micro-outlets as incipient estuarine systems - Does size and coastal connectivity count?

Micro-estuaries and micro-outlets are very small coastal systems that share some biotic and abiotic characteristics with larger temporarily closed estuaries, but differ in that the former have small localized catchments and limited connectivity with the marine environment. This multidisciplinary study reviews the first comprehensive investigation into the ecological structure of a subset of southern African micro-estuaries and micro-outlets, elucidating the biotic and abiotic components that characterize these systems. Furthermore, the microsystems (which have an open water area of less than 5 ha and a depth of less than 1 m) are compared with larger estuarine systems in the region, with the former representing one end of a continuum ranging from micro-outlets to large estuarine lakes and bays. In terms of the abiotic regime, the micro-outlets were dominated by oligohaline or freshwater conditions, while the micro-estuaries were mostly mesohaline. Relatively few freshwater-associated microalgae, zooplankton, macrozoobenthos and fish taxa were present in the micro-outlets, while a richer combination of freshwater and estuary-associated taxa dominated the micro-estuaries. Due to their small size and limited periods of abiotic stability, microsystems only serve as intermediate nursery areas for 0+ juveniles of estuary-associated marine fish which, after less than a year, migrate to nearby larger estuarine systems in order to complete their juvenile life stages. When compared to temporarily closed estuaries, predominantly open estuaries, estuarine lakes and estuarine bays, microsystems are depauperate in terms of typical estuarine and marine biota. However, further research is recommended on microsystems in southern Africa and globally, in order to define in greater detail their physico-chemical and biological characteristics.

Physiological responses of a juvenile marine estuarine-dependent fish (Family Sparidae) to changing salinity.

Estuaries are subject to high environmental variability coupled to tidal salinity shifts. Under restricted freshwater flow and prolonged drought conditions, salinity may exceed natural ranges and thus organisms may experience stressful hypersaline conditions. This study assessed the physiology of a juvenile marine estuarine-dependent species Rhabdosargus holubi (Family: Sparidae) under changing salinity to determine the impact on respiration and survival under shock and acclimatisation exposures. Oxygen consumption was not significantly different in the 2.5 to 45 salinity range and the interaction between temperature and salinity was not significant at the moderate levels tested. This confirmed the strong osmoregulatory capabilities of marine estuarine-dependent R. holubi. However, respiration was impacted at salinities of 55-62.5. The salinity tolerance ranges of R. holubi were expanded beyond those previously observed in the laboratory, to a maximum of 77 when fish were gradually exposed to daily 20% incremental changes. This indicated the ability to adapt to hypersaline conditions that occur gradually in anthropogenically altered estuaries which is an important aspect for management decisions regarding freshwater inputs. The adaptability of Rhabdosargus holubi to hypersaline conditions in estuaries may thereby ensure the maintenance of populations in the short term under certain environmental conditions, such as those currently occurring in a freshwater-scarce South Africa. However, in the long term and at more extreme salinities (> 55), the physiology of R. holubi and similar species may be compromised, thus placing the species at risk.

Natural nutrient enrichment and algal responses in near pristine micro-estuaries and micro-outlets.

Naturally-occurring pristine estuarine ecosystems are rare in modern environments due to anthropogenic encroachment. There are more than 100 outlets around the South African coast arising from streams flowing from small catchments close to the sea. Eight near natural systems were sampled seasonally over the period of a year to acquire baseline information on water quality and chlorophyll a status across a variety of algal guilds (benthic microalgae, phytoplankton and macroalgal cover). Albeit on a much smaller-scale, these systems represent natural surrogates of larger temporarily open/closed estuaries (TOCEs). Inorganic nutrients (ammonium, total oxidized nitrogen and soluble reactive phosphate), phytoplankton and microphytobenthos chlorophyll a, as well as macroagal percentage cover, were measured using standard methods. Algae showed a seasonal trend, with blooms of both micro- and macro-algae occurring during summer, with a dieback recorded in autumn. During summer, only one system had a phytoplankton peak in chlorophyll a above 20µgL-1, while the microphytobenthos concentrations in three of the systems were above 100mgm-2. Summer blooms of green filamentous macroalgae occurred in all four micro-outlets and in one micro-estuary. Using a linear mixed-effects modelling approach, significant drivers for algal growth related to temperature, nutrient conditions, light availability and water residence time, all of which are known to stimulate primary production. The results show that enrichment from natural sources display similar responses from primary producers to mesotrophic and/or eutrophic water bodies, with the exception that they revert to a natural state rather than continue into a degraded state as is the case in artificially enriched systems. This importantly demonstrates how larger temporarily/open closed estuaries, most of which are anthropogenically degraded, might have functioned under a former more balanced state. Some of these larger systems now respond to nutrient enrichment by exhibiting permanent cultural eutrophication.