Coleen L. Moloney

Monitoring the abundance of plastic debris in the marine environment

Plastic debris has significant environmental and economic impacts in marine systems. Monitoring is crucial to assess the efficacy of measures implemented to reduce the abundance of plastic debris, but it is complicated by large spatial and temporal heterogeneity in the amounts of plastic debris and by our limited understanding of the pathways followed by plastic debris and its long-term fate. To date, most monitoring has focused on beach surveys of stranded plastics and other litter. Infrequent surveys of the standing stock of litter on beaches provide crude estimates of debris types and abundance, but are biased by differential removal of litter items by beachcombing, cleanups and beach dynamics. Monitoring the accumulation of stranded debris provides an index of debris trends in adjacent waters, but is costly to undertake. At-sea sampling requires large sample sizes for statistical power to detect changes in abundance, given the high spatial and temporal heterogeneity. Another approach is to monitor the impacts of plastics. Seabirds and other marine organisms that accumulate plastics in their stomachs offer a cost-effective way to monitor the abundance and composition of small plastic litter. Changes in entanglement rates are harder to interpret, as they are sensitive to changes in population sizes of affected species. Monitoring waste disposal on ships and plastic debris levels in rivers and storm-water runoff is useful because it identifies the main sources of plastic debris entering the sea and can direct mitigation efforts. Different monitoring approaches are required to answer different questions, but attempts should be made to standardize approaches internationally.

Trophic flows in the southern Benguela during the 1980s and 1990s

Mass-balanced models of trophic flows in the southern Benguela ecosystem suggest a 10% increase in zooplankton biomass between the 1980s and the 1990s, in agreement with observed trends of increased zooplankton abundance off South Africa over the last few decades. Minimum hake biomass in balanced trophic models is substantially larger than survey and other model estimates,suggestingundersamplingofhakesinsurveysandunderestimationofjuvenilehakemortality.Modelbiomassandmean annual production of five important small pelagic fish groups were larger in the 1990s, and total catches were smaller than in the 1980s. Estimates of biomass per trophic level, transfer efficiencies, mixed trophic impacts and many other ecosystem attributes suggest that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. Because catches were lowerandmodelzooplanktonandsmallpelagicfishbiomasseswerelargerinthe1990s,theecosystemwaslesstightlyconstrained by predators (including fishers) and food availability than in the 1980s. Fishing took place at low trophic levels compared to other systems. Despite smaller total catches in the 1990s, fishing was ecologically more expensive (from higher trophic levels) during the 1990s than in the 1980s because snoek and hake catches were large. There was greater shared niche overlap of small pelagic fish predators in the 1990s than in the 1980s. Mean transfer efficiency was 12%. Transfer of biomass at trophic levels III–V appears to be more efficient in the southern Benguela than in other upwelling ecosystems. Primary production required to sustain catches in the southern Benguela ecosystem is 4% of total primary production, i.e. more similar to estimates for open ocean and coastal regions than for other upwelling or shelf systems averaging more than double this value. D 2002 Elsevier Science B.V. All rights reserved.

8 Resource and ecosystem variability, including regime shifts, in the Benguela Current System

Abstract Interannual and decadal-scale variability in abundance, distribution and biological characteristics are described for important living marine resources of the Benguela Current system including small pelagic fish, horse mackerel, hakes, snoek, rock lobster, Cape fur seals, Cape gannets and African penguins. Variability at the ecosystem level for the northern and southern subsystems is also described using trophodynamic indices that track structural changes in the ecosystem. Current understanding and analysis of observed variability in both resources and the ecosystem is reviewed, and the knowledge required for predicting resource and ecosystem variability and the causal factors that need to be considered are discussed. We highlight the need to improve understanding of the processes that are important in Benguela Current ecosystem, to identify what controls those processes, and to quantify such controls (particularly those acting on lower trophic levels) and the role of important species in the ecosystem. The kinds of predictions considered possible in the Benguela Current system are examined, and a series of steps is suggested to improve understanding of ecosystem and fisheries dynamics and to monitor key aspects of the ecosystem.

Ecological Speciation in South Atlantic Island Finches

Examples of sympatric speciation in nature are rare and hotly debated. We describe the parallel speciation of finches on two small islands in the Tristan da Cunha archipelago in the South Atlantic Ocean. Nesospiza buntings are a classic example of a simple adaptive radiation, with two species on each island: an abundant small-billed dietary generalist and a scarce large-billed specialist. Their morphological diversity closely matches the available spectrum of seed sizes, and genetic evidence suggests that they evolved independently on each island. Speciation is complete on the smaller island, where there is a single habitat with strongly bimodal seed size abundance, but is incomplete on the larger island, where a greater diversity of habitats has resulted in three lineages. Our study suggests that the buntings have undergone parallel ecological speciation.

Short-term variability during an anchor station study in the southern Benguela upwelling system: Phytoplankton dynamics

Abstract The temporal variability of the phytoplankton and the role of sinking in such variability was examined in response to environmental changes associated with coastal upwelling during a 27-day anchor station study in St Helena Bay on the South African west coast. Two phytoplankton blooms were observed, both of which were directly related to the intrusion of recently upwelled water of high nutrient concentration. Many of the observed phytoplankton changes corresponded to recognised stages of succession, as turbulence dissipated, whereas others resulted from sequential changes owing to changes in water-mass type. The system progressed from a high biomass diatom bloom in turbulent, nutrient rich water, to a flagellate community at much lower biomass levels in stratified, nutrient depleted water. Changes in the phytoplankton corresponded to changes in the vertical stability of the water column, the stratification index giving good qualitative prediction of the relative dominance of diatoms and flagellates. Phytoplankton community changes were unpredictable at the species level, but showed systematic trends in the dominance patterns of higher taxonomic levels such as diatoms, dinoflagellates and microflagellates. A number of changes in the species composition resulted from the interrelation between turbulence and the variable sinking rates of different components of the phytoplankton. Rapid sinking of diatom resting spores represented the transition from an active surface growing stage to a resting stage positioned in deeper water, thus ensuring the restoration of cells to the surface layer by restricted mixing events. Losses from the euphotic zone were nevertheless of limited importance to changes in the phytoplankton biomass. Natural mortality and breakdown of phytoplankton cells within the surface layers is thought to have been most important in accounting for the phytoplankton biomass decline.

Characterising and comparing the spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem were characterised by comparing their egg abundances with environmental variables measured concomitantly during two different survey programmes: the South African Sardine and Anchovy Recruitment Programme (SARP), which comprised monthly surveys conducted during the austral summers of 1993/94 and 1994/95; and annual pelagic spawner biomass surveys conducted in early summer (November/December) from 1984 to 1999. Eggs were collected using a CalVET net. Physical variables measured included sea surface temperature (SST), surface salinity, water depth, mixed-layer depth, and current and wind speeds; biological variables measured included phytoplankton biomass, and zooplankton biomass and production. Spawning habitat was identified by construction of quotient curves derived from egg abundance data and individual environmental variables, and relationships between these variables were determined using multivariate co-inertia analysis. SARP data showed that anchovy spawning was associated with cool water and moderate wind and current speeds, whereas sardine spawning was related to warmer water and more turbulent and unstable conditions (i.e. high wind speeds and strong currents) than for anchovy. SARP data also showed significant differences in selection of spawning habitat of the two species for all environmental variables. The relationship between anchovy egg abundance and salinity was strongly positive, but strongly negative with water depth, phytoplankton biomass and zooplankton production. Sardine egg abundance was strongly positively related to current speed. The spawner biomass survey data demonstrated that the spawning habitat of anchovy was characterised by warm water and high salinity, whereas sardine spawning was associated with cool water and low salinity. The survey data showed significant differences in spawning habitat selection by anchovy and sardine for SST, salinity and zooplankton biomass, but not for the other environmental variables. There was a positive relationship between anchovy egg abundance and SST, salinity and mixed-layer depth, and a negative relationship with water depth, phytoplankton biomass and zooplankton production. For sardine there was a strong positive relationship between egg abundance and current speed and wind speed. Differences in the results between the two survey programmes could be attributable to differences in their spatio-temporal coverage. Spawning habitats of anchovy and sardine appear to be substantially different, with anchovy being more specific than sardine in their preference of various environmental conditions.

Application of the sequential t-test algorithm for analysing regime shifts to the southern Benguela ecosystem

Long-term ecosystem changes, such as regime shifts, have occurred in several marine ecosystems world-wide. Multivariate statistical methods have been used to detect such changes. A new method known as the sequential t-test algorithm for analysing regime shifts (STARS) is applied to a set of biological state variables as well as environmental and anthropogenic forcing variables in the southern Benguela. The method is able to correct for auto-correlation within time-series by a process known as prewhitening. All variables were tested with and without prewhitening. Shifts that were detected with both methods were termed robust. The STARS method detected shifts in relatively short time-series and identified when these shifts occurred without a priori hypotheses. Shifts were generally well detected at the end of time-series, but further development of the method is needed to enhance its performance for auto-correlated time-series. Since 1950, two major long-term ecosystem changes were identified for the southern Benguela. The first change occurred during the 1960s, caused predominantly by heavy fishing pressure but with some environmental forcing. The second change occurred in the early 2000s, caused mainly by environmental forcing. To strengthen these findings, further analyses should be carried out using different methods.

Long-term decreases in persistent organic pollutants in South African coastal waters detected from beached polyethylene pellets

Polyethylene pellets provide a convenient means to monitor Persistent Organic Pollutants (POPs) in marine systems. Pellets collected between 1984 and 2008 at three South African beaches were analysed for PCB, HCH and DDT. Concentrations of all three POPs decreased over the last two decades, although this signal was less clear for PCBs, and further monitoring is needed to assess trends in this family of compounds. DDT concentrations at two sites were higher than previous records for southern Africa, but there is no evidence of a link to the ongoing use of DDT for malaria control. HCHs concentrations were lower than in pellets from the east coast of southern Africa, suggesting that this pesticide was mainly used in the eastern part of the region. Our study demonstrates the potential for International Pellet Watch to track temporal as well as geographical patterns in the abundance of POPs in marine environments.

Combined fishing and climate forcing in the southern Benguela upwelling ecosystem: an end-to-end modelling approach reveals dampened effects

The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N2P2Z2D2-OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.

A stoichiometric model relating growth substrate quality (C:N:P ratios) to N:P ratios in the products of heterotrophic release and excretion

Abstract A stoichiometric model is developed to analyze the influence of growth substrate element composition on the N:P ratios of heterotrophic excretion (true excretion, i.e. metabolic by-products) and release (excretion+feces production) products. The model uses units of C, N, and P, and depicts three types of heterotrophs: the copepods, cladocerans, and bacteria. Most parameters of the model are estimated from experimental data sets representative of these heterotrophs. Net growth efficiencies for N and P vary according to the element composition of growth substrates. The simulated N:P ratios for release and excretion products of copepods and cladocerans are compared to experimental data for heterotrophic regeneration. Results indicate that the simulated N:P ratio for excretion is more representative of the experimental measurements than that for release, especially when the growth substrate N:P ratio is high. Thus the model assumption of adjustable excretion could explain stoichiometric regulation of growth substrates by heterotrophs. Our model gives better fits to observations than two other similar models, mainly because of its ability to simulate the excretion N:P ratio. The C content of growth substrates did not influence stoichiometric excretion and release of N and P for mesozooplankton, but it was important for bacteria. Stoichiometric regeneration of nutrients by heterotrophs affects phytoplankton growth, with large organism-dominated ecosystems accentuating N limitation, whereas small organism-dominated ecosystems favor P limitation.