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Bottom-trawl fishery is known to cause major disturbances to marine sediments as the dragging of trawl gears across the seabed fosters sediment resuspension, which can lead to organic particle remineralization and release of benthic CO2 and nutrients into bottom waters. However, its effects on carbon cycling and biological productivity, especially in highly productive regions like the Benguela Upwelling System (BUS), are less well studied. Here, we simulated carbon (C) and nutrient pathways from the trawled coastal seabed to overlying water masses that are being upwelled into the sunlit surface within the BUS, using shipboard data on sea surface and water column characteristics and published benthic CO2 emission estimates from bottom-trawled sediments. The latter reports 4.35 and 0.64 Tg C year-1 to be released from the seabed into upwelling source waters after bottom trawling in the northern (NBUS) and southern (SBUS) subsystems, respectively. Based on these values, we estimated a corresponding nitrate (N) input of 1.39 and 0.47 µmol kg-1 year-1, enhancing source water nitrate concentrations by ~5% and ~2%. Trawl-induced nitrate input into the sunlit surface could support a new production of 3.14 and 0.47 Tg C year-1 in the NBUS and SBUS, respectively, recapturing only 2/3 of CO2 released after bottom trawling into biomass, mainly due to differences in stoichiometric C:N ratios between the sediment (~9) and surface biomass (Redfield, 6.6). The remaining benthic CO2 can thereby lead to an increase in surface CO2 concentration and its partial pressure (pCO2), impeding CO2 uptake of the biological carbon pump in the BUS by 1.3 Tg C year-1, of which 1 Tg C year-1 is emitted to the atmosphere across the northern subsystem. Our results demonstrate the extent to which bottom trawling may affect the CO2 storage potential of coastal sediments on a basin-wide level, highlighting the need to better resolve small-scale sediment characteristics and C:N ratios to refine trawl-induced benthic carbon and nutrient effluxes within the BUS. [ABSTRACT FROM AUTHOR]

Seasonal phytoplankton blooms are one of the key features of the productive northern Benguela upwelling system (nBUS), however they are not well described thus far. In this study twelve years (2000–2012) of in situ chlorophyll-a data from a monitoring transect off the Namibian coast were analysed to assess the long-term and seasonal variability in chlorophyll-a as a measure of phytoplankton biomass and the occurrence of phytoplankton blooms. On the shelf, low chlorophyll-a concentrations were identified in 2001/2002, 2005/2006, and 2011/2012. The concentrations on the shelf were highest in 2008/2009 and 2010/2011. Major phytoplankton blooms defined at chlorophyll-a concentrations > 18 mg m − 3 occurred in five of the 12 years (2002/2003, 2004/2005, 2008/2009, 2009/2010 and 2010/2011) while minor blooms (> 13 mg m − 3 ) occurred in almost every year. The calculated climatology of the chlorophyll-a time series revealed a clear seasonality. Three chlorophyll-a maxima typically develop inshore over the year: an austral winter peak (August), an early austral summer peak (December) and a late summer/autumn peak (April). The analysis of synoptic hydrographic, nutrient and wind data revealed three different forcing mechanisms that all initiate an influx of nutrients into the surface mixed layer. [ABSTRACT FROM AUTHOR]

Long-term data sets are essential to understand climate-induced variability in marine ecosystems. This study provides the first comprehensive analysis of longer-term temporal and spatial variations in zooplankton abundance and copepod community structure in the northern Benguela upwelling system from 2005 to 2011. Samples were collected from the upper 200 m along a transect at 20°S perpendicular to the coast of Namibia to 70 nm offshore. Based on seasonal and interannual trends in surface temperature and salinity, three distinct time periods were discernible with stronger upwelling in spring and extensive warm-water intrusions in late summer, thus, high temperature amplitudes, in the years 2005/06 and 2010/11, and less intensive upwelling followed by weaker warm-water intrusions from 2008/09 to 2009/10. Zooplankton abundance reflected these changes with higher numbers in 2005/06 and 2010/11. In contrast, zooplankton density was lower in 2008/09 and 2009/10, when temperature gradients from spring to late summer were less pronounced. Spatially, copepod abundance tended to be highest between 30 and 60 nautical miles off the coast, coinciding with the shelf break and continental slope. The dominant larger calanoid copepods were Calanoides carinatus, Metridia lucens and Nannocalanus minor. On all three scales studied, i.e. spatially from the coast to offshore waters as well as temporally, both seasonally and interannually, maximum zooplankton abundance was not coupled to the coldest temperature regime, and hence strongest upwelling intensity. Pronounced temperature amplitudes, and therefore strong gradients within a year, were apparently important and resulted in higher zooplankton abundance. [ABSTRACT FROM AUTHOR]

Abstract This paper examines variations in gender-specific body growth, morphology and reproduction of the bearded goby (Sufflogobius bibarbatus) across the Namibian shelf, built on previous work on male reproductive modes. The results indicate a spatial variation in the size composition, condition factor and maturation of gobies across the shelf area. Low oxygen (<0.5 ml O 2 L−1) did not hinder reproduction in the bearded goby and off Walvis Bay, maturing and mature females and males were found mostly at the outer shelf edge (150–200 m depth). The histological analysis of gonads validates the macroscopic scale applied for assessing maturity, and the mean number of maturing oocytes was from 690 to 1060 g−1 body weight. Males were smallest in the central area (where oxygen levels are known to be lowest), and relative condition increased latitudinally from north to south. The bearded goby displays clear sexual size dimorphism (males larger than females), and there is evidence that supports previous findings suggesting that males display alternative reproductive tactics. Assumed territorial males were older than assumed sneaker males, and all morphological measurements, except eye diameter, were larger (absolutely and relatively) in territorial males compared to sneakers. The morphologic measurements of sneakers did not differ from those of females suggesting female mimicry by sneakers. The role of unclassified males was less clear. The findings are discussed in view of environmental variation and behavioural ecology. Highlights • Oxygen is a major limiting environmental factor in the sea. • Severe hypoxia occur in the Benguela upwelling ecosystem. • The Goboiid Sufflogobius bibarbatus is a fish found adapted to these environments. • It displays sexual size dimorphism, and alternative reproductive tactics in males. • Low oxygen reduce growth, but do not hinder reproduction. [ABSTRACT FROM AUTHOR]