Browsing by Author "Carrasco, Nicola Kim."

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Ciliate-zooplankton epibiosis in Lake St Lucia.
(2018) Jones, Salome.; Vosloo, Andre.; Perissinotto, Renzo.; Carrasco, Nicola Kim.
Epibiosis is a symbiotic association of two organisms in which one species (epibiont) uses the surface of another species (basibiont or host) as an attachment substrate. An increasing number of studies are revealing that epibionts have mainly deleterious effects on crustacean meso-zooplankton (hereafter referred to simply as zooplankton) hosts. In spite of its widespread occurrence, there are very few studies in Africa that address epibiosis in the aquatic environment, particularly involving zooplankton as hosts. Epibiotic ciliates are often found attached to zooplankton in the St Lucia Estuary, in northan KwaZulu-Natal, South Africa. St Lucia is the largest estuarine lake in Africa and is globally recognized for its ecological importance. A study was conducted in St Lucia between 2015 and 2017, with the aim of determining: the identity of the epibiotic ciliates; their species-specific association with the zooplankton of St Lucia; the effects they have on their hosts and the environmental conditions that promote their proliferation. Based on live observations and images obtained from protargol staining and scanning electron microscopy, the epibiotic ciliates in the St Lucia Estuary were identified as the peritrich sessilid Epistylis sp. (Chapter 1). The results of the experimental study in Chapter 2 were that Epistylis sp. is species-specific, attaching only to the dominant calanoid copepod Pseudodiaptomus stuhlmanni (mainly adults) and that this relationship is host density dependent. Another finding of Chapter 2 was that Epistylis sp. exerts a negative effect on the survivorship of heavily covered P. stuhlmanni. The results of Chapter 3 revealed a low RNA content and RNA:DNA ratio in epibiont-hosting P. stuhlmanni compared with their non-hosting counterparts, which implies a compromised nutritional status of epibiont-hosting copepods. Laboratory-based experiments detailed in Chapters 4 and 5 revealed that Epistylis sp. is: a) unaffected by temperature; and b) favoured by salinities below 20 and organically rich turbidity within the range 250–500 NTU. Results obtained from monthly field observations throughout 2016 (Chapter 6) showed no correlation of Epistylis sp. with these physico-chemical parameters and with the abundance of P. stuhlmanni. The latter result may be due to the uncharacteristically low abundance of the host P. stuhlmanni during the sampling period (January–December 2016). Overall, findings of this study suggest that peritrich epibionts can substantially and negatively affect host species and that they have a complex, context-dependent relationship with environmental conditions. The ecological implications of ciliate-zooplankton epibiosis in the St Lucia Estuary and in similar systems are discussed.
Dietary dynamics of two key fish species in the St Lucia estuarine system, South Africa.
(2014) Dyer, David Clifford.; Perissinotto, Renzo.; Carrasco, Nicola Kim.
Among the 155 species of fish recorded so far in the St Lucia estuarine lake, Oreochromis mossambicus and Ambassis ambassis are the two most prominent. Although originally endemic to southern Africa, O. mossambicus is now one of the most widely distributed exotic fish species worldwide. Together with A. ambassis, they have become the dominant fish species in the St Lucia estuarine lake since the closure of the mouth in 2002 and are, therefore, a crucial component of the food webs throughout the system. After a decade dominated by dry and hypersaline conditions, the St Lucia system has changed dramatically in terms of prevailing environmental conditions, as a result of higher than average rainfall at the end of 2011 and the onset of a new wet phase at the start of 2012. In response, A. ambassis, which prefers lower salinity regimes, has expanded its distribution range throughout the estuarine lake. Stable δ¹⁵N and δ¹³C isotope analysis was used in conjunction with gut content analysis to elucidate the diet of these species at sampling localities spanning the geographical range of the system and determine whether these species shift their diet in response to environmental or climatic shifts. From both studies it is evident that from a temporal and spatial scale these two species adopt similar, yet very different, dietary tactics. Oreochromis mossambicus was shown to adopt a generalist feeding strategy, opportunistically feeding on dietary items that are available thus allowing this species to alter its diet according to the environment that it inhabits. Trophic positioning of this species was found to be controlled by salinity in St Lucia as dietary composition differed greatly between sites. In contrast, Ambassis ambassis displayed a more specialist dietary composition, feeding predominantly on zooplankton. However, this species also opportunistically supplements its diet with additional sources when available. Trophic position of A. ambassis was higher in the dry season owing to the increased productivity of the system during the wet season. The success and dominance of both species in the St Lucia system can therefore be attributed to their dietary strategies. Under extreme environmental conditions, O. mossambicus has the added advantage of its wide tolerance of different environmental conditions, particularly salinity, thus allowing it to proliferate.
Feeding dynamics of invasive (Tarebia granifera) and native (Melanoides tuberculata) freshwater gastropods.
(2020) Naidoo, Senine.; Carrasco, Nicola Kim.; Miranda, Nelson Augusto Fernandes.
Tarebia granifera has invaded several South African estuaries, outnumbering native gastropods such as Melanoides tuberculata. Comparative feeding dynamics can aid in estimating and understanding invasion impacts. This study aimed to explore the feeding dynamics of these gastropods with the use of two different approaches. Stable Isotope Analyses (SIAs) were used to investigate the dietary preferences and niche overlaps in the St Lucia Estuary, while Functional Response Experiments measured the relationship between the consumption rate of a food resource and its availability. These experiments were run both with and without the presence of heterospecific chemical cues (secondary metabolites secreted by either species) to determine whether chemical cues influence the functional responses of T. granifera and M. tuberculata. Mixed species experiments were also conducted to observe individual behaviour (active, inactive and feeding) over time and measure resource consumption when both species are placed together under high and low food availability. The SIA showed that both gastropods exhibited generalist diets, however, due to niche partitioning, there was no significant difference in their dietary niche overlap. In the FR experiments, both species exhibited Type II FRs and had similar feeding rates. However, in the presence of heterospecific chemical cues, M. tuberculata exhibited a higher feeding rate at high food availabilities, whereas T. granifera showed efficient feeding dynamics under limited food resources. In the mixed species experiments, T. granifera spent more time feeding under low food availability while M. tuberculata spent more time feeding under high food availability. This study showed that in certain conditions (e.g. high resource availability) the alien species did not always have a feeding advantage. A multiple method approach is recommended when assessing ecological impacts of invasive species.
Impact of turbidity on pseudodiaptomus stuhlmanni, a dominant copepod in Lake St Lucia, iSimangaliso Wetland Park.
(2015) Jones, Salome.; Perissinotto, Renzo.; Carrasco, Nicola Kim.; Vosloo, Andre.
Although the St Lucia Estuary is a Ramsar Wetland of International Importance, it has historically been subjected to human-accelerated ecological stressors. One of these is high turbidity resulting from excessive sediment inputs. Laboratory-based studies have revealed a negative turbidity effect on the feeding and mortality rate of two dominant zooplankton species, the mysid Mesopodopsis africana and the calanoid copepod Acartiella natalensis. The first aim of this study was to determine the effect of turbidity on the feeding, respiration, and mortality rate of another important calanoid, Pseudodiaptomus stuhlmanni. Although this species was negatively affected by high turbidity, it was substantially more tolerant than M. africana and A. natalensis. The second aim was to test the field response of the dominant St Lucia zooplankton to a silt plume-causing flood event that occurred in March 2014. As M. africana was not abundant in the system prior to this event, attention was paid to the copepods. The field response of A. natalensis and P. stuhlmanni were in good agreement with the findings from the laboratory-based experiments. The population of A. natalensis underwent an immediate, and sharp decline, whereas that of P. stuhlmanni only declined in April 2014, after a month of surviving in highly turbid waters. However, P. stuhlmanni also took longer to recover, but this may be attributed to the attachment of parasitic epibiotic ciliates to this species. Therefore, although to different degrees, turbidity negatively impacted the dominant St Lucia zooplankton species. Through its observed positive correlation with the parasitic ciliates, turbidity further suppressed the abundance of the most turbid-water tolerant species, P. stuhlmanni. The importance of carefully managing sediment loading in St Lucia is stressed, as the effect of turbidity on zooplankton likely has food web-wide consequences.
Zooplankton dynamics and ecophysiology in the St. Lucia Estuary, with emphasis on the dominant mysid Mesopodopsis africana.
(2011) Carrasco, Nicola Kim.; Perissinotto, Renzo.; Pillay, Deena.
The St. Lucia Estuary, Africa’s largest estuarine lake, is currently experiencing an unprecedented crisis related to freshwater deprivation. This has resulted in a reversed salinity gradient and drastically reduced water levels. These harsh environmental conditions, combined with the limited connection with the open ocean have lead to a loss of biodiversity in the system. The dominant zooplankton taxa include the copepods Pseudodiaptomus stuhlmanni and Acartia natalensis and the mysid Mesopodopsis africana. In March 2007, the closed-mouth state was briefly interrupted by an open-mouth phase, induced by a unique combination of extreme climatic events. With the incoming seawater, previously excluded marine taxa re-entered the system, increasing its diversity significantly. Salinity and temperature have been referred to as driving forces in aquatic ecosystems. The tolerance limits of the key mysid species were, therefore, investigated. Results showed that M. africana has some of the highest recorded upper salinity and temperature tolerances for a mysid. Because of their high biomass, mysids have the potential to affect microalgal standing stocks. Their grazing dynamics (in relation to autotrophic food availability) were investigated in two contrasting environments within the estuary. Ingestion rates and subsequently population grazing impacts on the total microalgal standing stocks were higher at the Mouth than at Charters Creek. This was attributed to the harsh environmental conditions in the latter region. Despite the lower ingestion rates exhibited here, these mysids seem capable of meeting their energetic requirements from a microalgal diet alone. Stable isotope data, though, show that they also utilise a heterotrophic diet. Results of the mixed model SIAR v 4 revealed the contribution of the different carbon sources to the diet of M. africana. Most unique was this mysid’s ability to modify its diet on both short temporal and spatial scales. Resource utilization between the dominant taxa was also compared. All three taxa appear to be opportunistic feeders, capable of incorporating a number of food items in their diet. Between food partitioning, predator avoidance strategies, and their common ability to survive in highly dynamic environments, these species are capable of co-existing, and together contribute to the overall resilience so far shown by the system.
Zooplankton dynamics during a regime shift in the St Lucia Estuary, South Africa.
(2021) Govender, Merusha.; Carrasco, Nicola Kim.
The St Lucia Estuary is the largest estuarine lake in Africa and forms part of the iSimangaliso World Heritage Site however historical anthropogenic changes, including the separation of the Mfolozi River from the St Lucia system, have exacerbated periodic drought conditions. A Global Environment Facilitated (GEF) funded project was carried out (June 2016 to July 2017) to relink the Mfolozi River and St Lucia Estuary to allow unimpeded flow of freshwater into the St Lucia Estuary that would alleviate drought conditions and ideally restore natural mouth functioning. This reconnection, combined with increased precipitation from 2017 onwards, initiated a transition to a new wet phase in the estuary. Zooplankton form an essential link between primary producers and higher trophic levels, and they are important indicators of change in aquatic ecosystems. This study aimed to firstly record the changes in the zooplankton community before, during and after the restoration project in the mouth region from February 2015 to November 2018. The zooplankton community structure at the mouth differed significantly before and after the restoration project. Estuarine resident taxa including the mysid Mesopodopsis africana, the copepods Acartiella natalensis and Oithona brevicornis were absent post project completion as mainly freshwater taxa dominated. Both total suspended solids and salinity were identified as major determinants of the zooplankton community structure at the mouth region. Secondly, the study aimed to investigate the system-wide impacts by monitoring the zooplankton community at 5 representative sites within the system as it transitioned from a hypersaline state to a freshwater state from February 2015 to November 2017. Following the reconnection, water levels increased, and salinity levels decreased throughout the lake system, however the reversed salinity gradient persisted with a mean salinity of 23 recorded in the upper reaches of the lake system in 2017. There was a clear shift in the zooplankton community to freshwater taxa such as cyclopoids, rotifers and cladocerans. Resident taxa declined in abundance, as Oithona brevicornis and Acartiella natalensis were virtually absent in 2017, possibly due to low salinity, or increased competition from freshwater taxa. The calanoid copepod Pseudodiaptomus stuhlamnni was still present throughout the lake system (in lower abundance) and the mysid Mesopodopsis africana was restricted to the upper reaches which served as refuge from the low salinity areas. While the increased freshwater input is positive, it has not yet been substantial enough to initiate a mouth breaching event that would flush the system of the excessive silt build-up. Furthermore, continuation of this already extended wet phase could force the exclusion of the estuarine resident zooplankton.