The zooplankton community of the main channel of the Mngazana estuary was investigated on six occasions over one year. Spatial and temporal variability in zooplankton abundance and distribution were sampled using two slightly modified WP2 plankton nets of 200 μm mesh and 57 cm mouth diameter. Nets were fitted with calibrated Kahlsico 005 WA 130 flow meters. A set of environmental variables were also measured at each site. The water column of the main channel of Mngazana estuary was stratified in summer but well mixed in winter. Temperature progressively increased from the lower stations to the upper reaches of the estuary. Maximum vertical temperature differences occurred in the middle estuarine reaches. Salinity progressively decreased from the lower reaches to the upper reaches, but was generally above 26 from Station 3 to Station 8. Only during November 2005, were salinity readings low due to heavy rains. Summer salinity values were always lower than the winter salinity values as a result of summer rainfall. A total of 76 zooplankton taxa were identified. The estuarine copepods Acartia natalensis and Pseudodiaptomus hessei dominated the assemblage, with maximum abundances in the middle to upper reaches. Acartia natalensis reached high densities in winter (> 50 000 m-3) although lowest abundances were recorded when maximum rainfall was received (November 2005). Wooldridge (1977) and Deyzel (2004) recorded maximum densities during summer. Pseudodiaptomus hessei reached high densities (> 17 000 m-3) during the highest rainfall month. Previous studies indicated that this pioneer species is able to recover quickly after a flood and subsequently increase rapidly in numbers. The mysid, Mesopodopsis africana reached high densities (> 5 000 m-3) in the middle estuarine reaches during summer. Ichtyofauna, brachyura and anomura were important contributors to the merozooplankton component of the community. The maximum number of species was recorded during winter in the lower estuarine reaches, when vertical differences in salinities were minimal.

[Truncated abstract] The role of marine microzooplankton in aquatic food webs has been studied in most regions of the world’s oceans, with the exception of the subtropical/temperate eastern Indian Ocean. This thesis addresses this gap in knowledge by investigating microzooplankton from five stations on a cross continental shelf transect and in two mesoscale features ∼300 km offshore of south west Western Australia. My primary focus was to measure and evaluate microzooplankton community change over space and time and their impact on phytoplankton on a cross shelf transect, sampling five stations from February 2002 December 2004 as part of a large multidisciplinary investigation into the pelagic ecosystem on the shelf (Chapter 2). This transect was named the Two Rocks transect. I also investigated an eddy pair (Chapter 5), which had originated from water in the vicinity of the Two Rocks transect, also undertaken as part of a larger study, investigating biophysical coupling within mesoscale eddies off south west Western Australia . . . The distribution of mixotrophic cells differed across the transect. Those mixotrophs that use photosynthesis as their primarily energy source exploited nutrient limited conditions inshore consuming particles, while mixotrophs that are primarily heterotrophic survived low prey conditions offshore by photosynthesizing. In the eddies, the grazing behaviour of microzooplankton was dependent on the specific phytoplankton assemblage in each eddy. The warm core eddy had a resident population of diatoms that were consumed by heterotrophic dinoflagellates present in high numbers. The cold core eddy had a warm cap which prevented upwelled water reaching the surface, resulting in stratification and a very active microbial food web, particularly in the surface.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Biology; and the Woods Hole Oceanographic Institution), 2006. / Includes bibliographical references (p. 297-311). / The physical and biological forces that drive zooplankton distribution and patchiness in an antarctic continental shelf region were examined, with particular emphasis on the Antarctic krill, Euphausia superba. This was accomplished by the application of acoustic, video, and environmental sensors during surveys of the region in and around Marguerite Bay, west of the Antarctic Peninsula, in the falls and winters of 2001 and 2002. An important component of the research involved the development and verification of methods for extracting estimates of ecologically-meaningful quantities from measurements of scattered sound. The distribution of acoustic volume backscattering at the single frequency of 120 kHz was first examined as an index of the overall biomass of zooplankton. Distinct spatial and seasonal patterns were observed that coincided with advective features. Improved parameterization was then achieved for a theoretical model of Antarctic krill target strength, the quantity necessary in scaling measurements of scattered sound to estimates of abundance, through direct measurement of all necessary model parameters for krill sampled in the study region and survey period. / (cont.) Methods were developed for identifying and delineating krill aggregations, allowing the distribution of krill to be distinguished from that of the overall zooplankton community. Additional methods were developed and verified for estimating the length, abundance, and biomass of krill in each acoustically-identified aggregation. These methods were applied to multi-frequency acoustic survey data, demonstrating strong seasonal, inter-annual, and spatial variability in the distribution of krill biomass. Highest biomass was consistently associated with regions close to land where temperatures at depth were cool. Finally, the morphology, internal structure, and vertical position of individual krill aggregations were examined. The observed patterns of variability in aggregation characteristics between day and night, regions of high versus low food availability, and in the presence or absence of predators, together reinforced the conclusion that aggregation and diel vertical migration represent strategies to avoid visual predators, while also allowing the krill access to shallowly-distributed food resources. The various findings of this work have important implications to the fields of zooplankton acoustics and Antarctic krill ecology, especially in relation to the interactions of the krill with its predators. / by Gareth L. Lawson. / Ph.D.

The high productivity of Eastern Boundary Upwelling Ecosystems (EBUE), some of the most productive ecosystems in the globe, is attributed to the nutrient rich waters brought up through upwelling. Climate change scenarios for coastal upwelling systems, predict an intensification of coastal upwelling winds. Associated with intensification in upwelling are biogeochemical changes such as ocean hypoxia and ocean acidification. In recent years, the California Current System (CCS) has experienced the occurrence of nearshore hypoxia and the novel rise of anoxia. This has been attributed to changes in the intensity of upwelling wind stress. The effects of some of the more severe hypoxia and anoxia events in the CCS have been mass mortality of fish and benthic invertebrates. However, the impacts on zooplankton in this system are not known. Meroplankton, those organisms which have a planktonic stage for only part of their life cycle, are an important component of zooplankton communities. The larval stage of benthic invertebrates forms an important link between benthic adult communities and planktonic communities. Larvae serve to disperse individuals to new locations and to link populations. They are also food for fish and planktonic invertebrates. This important life stage can spend long periods in the plankton (from days to months) where environmental conditions can affect larval health, subsequent settlement and recruitment success, and juvenile health. This research assesses the role of hypoxia and larval survivorship, and the relationship between individual abundance and community structure of larvae to environmental factors in the field. In laboratory experiments (Chapter 2), a suite of 10 rocky intertidal invertebrate species from four phyla were exposed to low oxygen conditions representative of the nearshore environment of the Oregon coast. Results revealed a wide range in tolerances from species with little tolerance (e.g. the shore crab Hemigrapsus oregonensis) to species with high tolerance (e.g. the California mussel Mytilus californianus). The differential responses across larvae to chronic hypoxia and anoxia potentially could affect their recruitment success and consequently, the structure and species composition of intertidal communities. Field studies (Chapter 3 & 4) explore the relationship between environmental variables and larval abundance and community structure. Chapter 3 focuses on broad taxonomic groups, while Chapter 4 focuses on larval decapods in particular. Fine focus was devoted to decapod larvae, due to laboratory findings of heightened sensitivity to hypoxia of decapod crabs. A finding that is also supported in the literature. The goal of field studies was to identify the environmental parameters that structure meroplankton and larval decapod communities and identify which of these parameters play a significant role in influencing larval abundance. A number of environmental variables contributed to meroplankton assemblage structure and larval decapod assemblage structure. These included distance from shore, depth, date, upwelling intensity, dissolved oxygen, and cumulative wind stress. Some of these factors occurred frequently in larval abundance models. In Chapter 3, individual abundance across broad taxonomic groups was most commonly explained by upwelling intensity while in Chapter 4, individual abundance of different decapod species was explained by cumulative wind stress, which is a proxy for upwelling intensity. The prominent role of upwelling related factors in explaining individual abundance is important considering climate change projections of an increased intensification of upwelling winds in EBUE. / Graduation date: 2012 / Access restricted to the OSU Community at author's request from Jan. 6, 2012 - Jan. 6, 2013

xvii, 175 leaves : ill. ; 29 cm Notes Typescript Thesis (Ph. D.)--University of Oregon, 2002 Includes vita and abstract Includes bibliographical references (leaves 150-175) Another copy on microfilm is located in Archives

Larval fish and zooplankton assemblages were studied in nine south and west coast estuaries in the cool-temperate and the cool/warm-temperate boundary region between June 2003 and March 2004. This study served to provide new information on previously unstudied estuaries and expand on existing knowledge of larval fish and zooplankton assemblages associated with various estuary types. The south and west coast estuaries sampled in this study showed lower salinities (12.2 - 13.7), lower water temperatures (14.5 - 16.9 °C) and higher turbidities (k = 0.02 - 0.04) in winter and spring while higher salinities (21.7 - 21.8), higher water temperatures (21.7 - 23.1°C) and lower turbidities (k <0.02) were observed in summer and autumn. Mean winter and summer water temperatures in estuaries were lower than those observed in warm-temperate and subtropical systems by other researchers. A total of 49274 larval fishes were caught, comprising 9 orders, 20 families, 29 genera and 47 taxa. The clupeid Gilchristella aestuaria (78.8 %) dominated the larval fish assemblages and occurred in all estuaries. The majority (70 %) of identified species are endemic to southern Africa and 96.4 % of larval fishes caught were estuary-resident species. The zooplankton study yielded a total of 44 taxa, comprising 7 phyla, >20 orders and >35 families. The copepod Pseudodiaptomus hessei dominated (59 %) the zooplankton and occurred in similar densities to those observed in other South African estuaries. Larval fish and zooplankton varied across seasons, peaking simultaneously in summer although zooplankton showed additional density peaks during the closed phase of some estuaries. Both plankton components were more abundant in the oligohaline and mesohaline zones within the estuaries. Freshwater input, estuary type and the biogeography of the area influenced the composition and structure of larval fish and zooplankton assemblages in these estuaries. The findings suggest that the estuaries are functioning as successful breeding areas for the larvae of endemic estuary-resident fish species and that these estuaries have to be managed to ensure an adequate freshwater supply to maintain the biological integrity of the ecosystem, specially the maintenance of the highly productive River-Estuary Interface (REI) regions.

The larval fish and zooplankton assemblages were studied in the permanently open Sundays Estuary on the south-east coast of South Africa, using standard boat-based plankton towing methods. A total of 8174 larval and early juvenile fishes were caught, representing 12 families and 23 taxa. The Clupeidae, Gobiidae and Blenniidae were the dominant fish families. Common species included Gilchristella aestuaria, Caffrogobius gilchristi, Omobranchus woodi, Liza dumerilii, Glossogobius callidus and Myxus capensis. Estuarine resident species (Category I) predominantly in the preflexion developmental stage, dominated the system. A total of 19 zooplankton taxa were recorded. Copepoda dominated the zooplankton community. Dominant species included Pseudodiaptomus hessei, Acartia longipatella, Halicyclops sp., Mesopodopsis wooldridgei, and the larvae of Paratylodiplax edwardsii and Hymenosoma orbiculare. Mean larval fish density showed similar trends seasonally, spatially and across salinity zones, with mean zooplankton density in the Sundays Estuary. Gut content analysis of five larval fish species: Gilchristella aestuaria, Pomadasys commersonnii, Monodactylus falciformis, Myxus capensis and Rhabdosargus holubi, revealed species specific diet and prey selection. Although larval fish diet contained a variety of prey items, guts were dominated by P. hessei, chironomid larvae, Corophium triaenonyx, copepod eggs and insect larvae. Physico-chemical drivers and the interactions between these two plankton communities provide information that enables a more holistic view of the dynamics occurring in the Sundays Estuary planktonic ecosystem.