Physical processes contributing to harmful algal blooms in Saldanha Bay, South Africa

The study synthesises current understanding of the predominant physical processes responsible for the seasonality of harmful algal blooms, notably Alexandrium catenella and Dinophysis spp., in the nearshore environmentof Saldanha Bay on the west coast of South Africa. Saldanha Bay is one of the few naturally sheltered areas on the South African coastline suitable for in situ shellfish farming and is the major site for the productionof black mussel Mytilus galloprovincialis in South Africa. Mussel farming started there in 1985 and the present level of production is some 2 700 tons per annum. Since 1994, disruption of harvesting as a result of the presence of harmful algal species has been a regular late-summer phenomenon. Toxic blooms that are ultimately advected into the bay develop on the continental shelf to the north between 32°S and St Helena Bay, a region characterized by favourable conditions for dinoflagellate growth and circulation patterns that facilitate build-up of intenseblooms during late summer. Offshore dinoflagellate populations are advected shorewards and polewards in response to relaxation of upwelling at the Namaqua cell to the north. Dinoflagellate blooms are advected south from the southern Namaqua shelf during upwelling relaxation. Under such conditions, the gyre south of Elands Bay moves offshore and a barotropic flow past Cape Columbine is established. Evidence suggests that the nearsurface component of the flow occurs as a sudden “flood” event. These dinoflagellate-containing shelf waters are in turn advected into Saldanha Bay when upwelling relaxes, when the density gradient between the bay and the shelf drives surface inflow and bottom water outflow. These flows are reversed with the resumption of upwellingover the shelf, resulting in intrusion and entrainment of bottom water and surface outflow. Entrainment dictates that the bay acts as a net importer of bottom water and net exporter of surface waters over a synoptic cycle. Thissystem of exchange between Saldanha Bay and the shelf curtails the duration and severity of toxic episodes in the bay relative to the shelf

Seasonal Depletion of the Dissolved Iron Reservoirs in the Sub-Antarctic zone of the Southern Atlantic Ocean

Seasonal progression of dissolved iron (DFe) concentrations in the upper water column were examined during four occupations in the Atlantic sector of the Southern Ocean. DFe inventories from euphotic and aphotic reservoirs decreased progressively from July to February, while dissolved inorganic nitrogen (DIN) decreased from July to January with no significant change between January and February. Results suggest that between July and January, DFe loss from both euphotic and aphotic reservoirs were predominantly in support of phytoplankton growth (Iron to carbon (Fe:C) uptake ratio of 16±3 μmol mol‐1) highlighting the importance of the “winter DFe‐reservoir” for biological uptake. During January to February, excess loss of DFe relative to DIN (Fe:C uptake ratio of 44±8 μmol mol‐1 and aphotic DFe loss rate of 0.34±0.06 μmol m‐2 d‐1) suggests that scavenging is the dominant removal mechanism of DFe from the aphotic, while continued production is likely supported by recycled nutrients. Plain Language Summary Trace metal iron is one of the limiting nutrients for primary productivity in the Southern Ocean; however the relative importance of seasonal iron supply and sinks remains poorly understood, due to sparse data coverage across the seasonal cycle and lack of high‐resolution dissolved iron (DFe) measurements. Here, we present four “snap‐shots” of DFe measurements at a single station in the south‐east Southern Atlantic Ocean (one in winter and three in late spring‐summer), to address the seasonal evolution of DFe and dissolved inorganic nitrogen (DIN) concentrations within the biologically active sunlit and subsurface reservoirs. We observed a seasonal depletion of DFe inventories from July‐February, while DIN inventories decreases from July‐January with no concomitant changes between January‐February. This suggests that, in addition to biological uptake in the sunlit layer, the observed decrease in DFe inventories below this (relative to DIN) is driven by aggregation and incorporation of iron particles into larger "marine snow" sinking particles, while nutrient recycling is driving the observed continuation of primary productivity during late summer. Our results provide insight into seasonal change of DFe availability in different reservoirs where interplay between removal and supply processes are controlling its distributions and bioavailability to support upper surface primary production

Strengthening seasonal marine CO2 variations due to increasing atmospheric CO2

The increase of atmospheric CO2 (ref. 1) has been predicted to impact the seasonal cycle of inorganic carbon in the global ocean2,3, yet the observational evidence to verify this prediction has been missing. Here, using an observation-based product of the oceanic partial pressure of CO2 (pCO2) covering the past 34 years, we find that the winter-to-summer difference of the pCO2 has increased on average by 2.2 ± 0.4 μatm per decade from 1982 to 2015 poleward of 10° latitude. This is largely in agreement with the trend expected from thermodynamic considerations. Most of the increase stems from the seasonality of the drivers acting on an increasing oceanic pCO2 caused by the uptake of anthropogenic CO2 from the atmosphere. In the high latitudes, the concurrent ocean-acidification-induced changes in the buffer capacity of the ocean enhance this effect. This strengthening of the seasonal winter-to-summer difference pushes the global ocean towards critical thresholds earlier, inducing stress to ocean ecosystems and fisheries4. Our study provides observational evidence for this strengthening seasonal difference in the oceanic carbon cycle on a global scale, illustrating the inevitable consequences of anthropogenic CO2 emissions

Unusual circulation patterns of the rias baixas induced by minho freshwater intrusion (NW of the Iberian Peninsula)

The Minho River, situated 30 km south of the Rias Baixas, is the most important freshwater source flowing into the Western Galician coast (NW of the Iberian Peninsula). The buoyancy generated by the Minho estuarine plume can reverse the normal circulation pattern inside the Rias Baixas affecting the exchange between the Rias and the ocean, changing the input of nutrients. Nevertheless, this inversion of the circulation patterns is not a well-monitored phenomenon. The only published results based on in situ data related to the presence of the Minho River plume inside the Rias de Vigo and Pontevedra correspond to an event measured on spring 1998. In this case unexpectedly higher inflow surface current velocities were found at the Ria de Pontevedra, located further away from Minho River. Thus, the main aim of this study is to research the main factors inducing this unusual pattern on the circulation of the Rias de Vigo and Pontevedra. A numerical model implementation of MOHID previously developed, calibrated, and validated for this coastal area was used. Several scenarios were performed in order to explain the individual effect of the Minho River, rivers discharging into each Rias, and estuarine morphology changes. According to the model results, the Minho River discharge is a key factor in the establishment of the negative circulation, while small rivers inside the Rias slightly attenuate this circulation. The negative circulation was stronger in Ria de Pontevedra independently of the distance of this coastal system from the Minho River mouth, showing that morphologic estuarine features are the main factor justifying the different local circulation patterns

A structured ecosystem-scale approach to marine water quality management

Activities and developments in the coastal zone, and in adjacent catchments, pose an increasing threat to the sustainability of the natural and socio-economic goods and services supplied by marine ecosystems. Governing authorities have had to develop new policies to promote environmentally responsible and sustainable development practices, either through legislation and/or incentive mechanisms. These, in turn, created the need for holistic and integrated frameworks within which to design and implement environmental management programmes. A structured ecosystem-scale approach for the design and implementation of marine water quality management programmes developed by the CSIR (South Africa) in response to recent advances in policies and legislation pertaining to sustainable utilisation of Southern Africa's marine environment is discussed. The framework provides an integrated scientific base within which to set, for example, wastewater emission targets, taking into account ecosystem process complexity. It also aims to support and stimulate local stakeholder empowerment and involvement. Water SA Vol.32 (4) 2006: pp.535-54

&#948 15N as a tool to demonstrate the contribution of fish-waste-derived nitrogen to an ulva bloom in Saldanha Bay, South Africa

This study uses stable isotope ratios of nitrogen (ä15N) to test the hypothesis that a bloom of the green seaweed Ulva lactuca, which occurred in Saldanha Bay, South Africa, in summer 1993/94 was linked to an adjacent discharge of nitrogen from pelagic fish processing waste. It is suggested that only two significant sources of new nitrogen were available to the Ulva: the natural nitrate flux from coastal upwelling and the fish factory nitrogen effluent. A significant difference (1.9‰) was found in the mean ä15N values between Ulva samples from a control site at Langebaan Lagoon (8.9‰) and those from Saldanha Bay (10.8‰). The latter value, which is relatively enriched in 15N, is consistent with the view that the nitrogen taken up in Saldanha Bay originated from a trophic position corresponding to pelagic fish in the southern Benguela system. The ä15N values from Ulva at the control site are consistent with nitrogen originating from the natural oceanic nitrate pool. It issuggested that this stable isotope method is useful in linking the causes and effects of eutrophication

A Note On Wind-Driven Replacement Flow of the Bottom Layer in Saldanha Bay, South-Africa - Implications for Pollution

The unexpected effect of a synoptic-scale event (passage of a cold front) on the advection of water across the mouth of Small Bay, Saldanha Bay, is recorded. From a current-meter record, it is shown how the strength (> 15m.s-1) and direction (NNE) of the wind relative to the mouth set up a stratified shear flow whereby the out-flowing surface water is replaced by in-flowing bottom water. The potential environmental implications for this type of event are discussed

Mesoscale features and phytoplankton biomass at the GoodHope line in the Southern Ocean during austral summer

Two sets of high-resolution subsurface hydrographic and underway surface chlorophylla (Chl a) measurements are used, in conjunction with satellite remotely sensed data, to investigate the upper layer oceanography (mesoscale features and mixed layer depth variability) and phytoplankton biomass at the GoodHope line south of Africa, during the 2010–2011 austral summer. The link between physical parameters of the upper ocean, specifically frontal activity, to the spatially varying in situ and satellite measurements of Chl a concentrations is investigated. The observations provide evidence to show that the fronts act to both enhance phytoplankton biomass as well as to delimit regions of similar chlorophyll concentrations, although the front–chlorophyll relationships become obscure towards the end of the growing season due to bloom advection and ‘patchy’ Chl a behaviour. Satellite ocean colour measurements are compared to in situ chlorophyll measurements to assess the disparity between the two sampling techniques. The scientific value of the time-series of oceanographic observations collected at the GoodHope line between 2004 to present is being realised. Continued efforts in this programme are essential to better understand both the physical and biogeochemical dynamics of the upper ocean in the Atlantic sector of the Southern Ocean. Keywords: chlorophyll a, mixed layer depth, ocean frontsAfrican Journal of Marine Science 2012, 34(4): 511–52