Periods Of Upwelling Research Articles

Influences of meteorological conditions, runoff, and bathymetry on summer thermal regime of a Great Lakes estuary

To better understand the primary drivers of the thermal regime in a Great Lakes estuary, and their implications for local biota, water temperature variations in the Milwaukee Estuary of Lake Michigan were studied between July and October of 2019 using a network of 25 sensors at 18 locations. Like Lake Michigan, the estuary was thermally stratified July to October, and historically dredged channels with water depths greater than 6 m allowed for the free movement of cold lake water into the estuary. However, temperatures in the estuary fluctuated rapidly both spatially and temporally, reflecting lake temperature fluctuations driven by changing meteorological conditions. Lake-driven upwelling and downwelling events influenced water temperature more than tributary contributions. Periodic upwelling and downwelling events caused temperature changes by up to 15 °C in less than 24 h. Nearshore upwelling events occasionally allowed cold, hypolimnetic lake water to reach tributary portions of the estuary beyond dredged areas, while downwelling events disrupted thermal stratification and caused the deep, dredged portions of the estuary to exceed 20 °C. Thermal impacts from these events were especially noticeable in transition zones between dredged and not dredged channels less than 2 m deep. The warming effects from downwelling persisted longer inside the estuary – up to two weeks – than cooling effects from upwelling, which typically lasted a few days. The predominantly lake-driven, rapid summer water temperature fluctuations observed in the Milwaukee Estuary serve as an important consideration in habitat restoration activities happening in Great Lakes estuaries.

Impacts of climate-driven upwelling intensification on the microbiota and health of cultivated scallops

The scallop Argopecten purpuratus plays a pivotal role as an economic resource cultivated in regions where climate change may intensify the coastal upwelling. This process has the potential to impact host-microbiota interactions. Hence, we conducted a field study to investigate the effects of varying upwelling intensities on hemolymph and gill microbiota dynamics (utilizing 16S rRNA gene sequencing), as well as the pathological state of A. purpuratus maintained in culture systems. Helicobactereaceae and Arcobactereaceae emerged as the prevailing families within both the hemolymph and gills. In contrast to the period of weak upwelling, the phase characterized by intensified upwelling revealed notable shifts in the abundance of bacterial community members, thereby leading to implications for the functional potential of the hemolymph microbiota. Notably, environmental factors played a substantial role in shaping the microbial community, accounting for 82–96% of the observed variation. Among these factors, wind, oxygen, and temperature exerted the most influence. During intensified upwelling, the abundance of Arcobactereaceae increased from 27% to 36% within gills and from 43% to 55% within hemolymph. Conversely, Helicobactereaceae decreased, particularly within gills, from 15% to 8% during the same period. Significantly, bacteria with fermentation capabilities, such as Colwellia, along with those involved in sulfur cycle processes, such as Arcobactereaceae, Desulfobulbaceae, and Desulfobacteraceae, exhibited an elevated representation during the period of intensified upwelling. This suggests potential nutritional advantages for A. purpuratus. However, the intense upwelling phase also saw an overrepresentation of potentially pathogenic bacteria for mollusks, including Francisella halioticida, Vibrio, and Photobacterium. Moreover, histopathological examination revealed heightened hemocytic infiltration, the presence of granulomatous “swirl” lesions, and a potential Apicomplexan parasite during this intensified upwelling period. These findings underscore the importance of the adaptability of host-microbiota interactions and the potential risks of pathogenicity for cultivated A. purpuratus within the context of ongoing and future environmental changes. Therefore, continuous monitoring of pathogen emergence and immune status evaluation during intensified upwelling periods are crucial for successful A. purpuratus aquaculture.

Summer Chlorophyll-a Increase Induced by Upwelling off the Northeastern Coast of Hainan Island, South China Sea

There are generally high chlorophyll-a concentrations (Chl-a) where upwelling is prevalent. High Chl-a is often observed in upwelling areas of the northeastern coast of Hainan Island during the summer. Using the satellite remote sensing data, including sea surface temperature, sea surface wind and Chl-a data from 2009 to 2022, we analyze the spatial-temporal and inter-annual variation of Chl-a on the northeastern coast of Hainan Island. Then, the possible influence of environmental factors on Chl-a are further examined by using satellite data, as well as Ekman transport and Ekman pumping velocity derived from the wind products. Finally, the key factors affecting the changes of Chl-a are discussed by correlation analysis. The results show the significant interannual variation of Chl-a in the region, with the maximum of summer Chl-a during the prevalent period of upwelling. The correlation analyses reveal that there is a higher correlation coefficient between the summer Chl-a and the upwelling index (i.e., upwelling regional temperature anomaly), suggesting the role played by upwelling in the summer high Chl-a is more important than the other environmental factors. It is speculated that the summer Chl-a increase is not only influenced by wind-induced upwelling but also by the upwelling caused by tidal mixing, large-scale circulation, topographic changes, and typhoon events.

Heterotrophy, microbiome, and location effects on restoration efficacy of the threatened coral Acropora palmata

The iconic and threatened Caribbean coral, Acropora palmata, is an essential reef-ecosystem engineer. Understanding the processes underpinning this coral’s survival and growth is essential to restoring this foundational species. Here, we compared replicate A. palmata colonies transplanted along 350 km of Florida’s offshore coral reef to determine holobiont and/or environmental variables that predict transplant success. We found a west-to-east gradient in coral physiology coupled with site-specific coral-associated microbiomes. Interestingly, no variables were linked to coral genet. Our results suggest that the unique oceanographic conditions with periodic upwelling events in the Dry Tortugas provide corals with greater opportunity for heterotrophy that in turn enhances coral growth and survivorship, and positively influences the microbiome. Our findings indicate that restoration efforts in the Dry Tortugas, and other places exhibiting higher food availability, could be most effective for A. palmata.

The Effects of the Indonesian Throughflow, River, and Tide on Physical and Hydrodynamic Conditions During the Wind- Driven Upwelling Period North of the Aru Islands

A three-dimensional baroclinic nonlinear numerical model—the Hamburg Shelf Ocean Model (HAMSOM)—was applied to investigate the effects of the Indonesian throughflow (ITF), river runoff, and tidal forcing on circulation during the southeast monsoon period (July 2004) north of the Aru Islands by conducting different sensitivity runs. It was found that the Ekman transport over the continental slope of the Sahul Shelf was the main factor that causes upwelling north of the Aru Islands, and this was suggested to be one of the main factors behind the surface water in the research area being relatively colder and saltier than the surrounding waters. The influence of South Pacific Subtropical Water (SPSW) on the surface water was indicated by the high surface salinity of waters within the internal salinity maximum layer. The results also suggested that onshore subsurface currents over the slope were induced not only by offshore surface currents over the slope but also by the ITF. By considering the eastern ITF route, river runoff and tidal forcing were also found to contribute significantly to the erosion of the salinity maximum (approx. 0.25) within the Halmahera Sea, thereby reducing sea surface salinity north of the Aru Islands. Furthermore, it was proposed that river runoff from the western coast of Papua Island contributed to intensified cross-shelf circulation over the continental slope. These conditions were related to enhancing vertical viscosity forces in the surface waters induced by stronger stratification as an impact of river inclusion in the simulation.

Assessment of potential variability of cadmium and copper trace metals using hindcast estimates.

Trace metals are vital to primary productivity and play an essential role as main components in regulating oceanic biogeochemical cycles. Dissolved and particulate trace metals within the water column may vary due to primary production, temperature, and nutrient changes, factors that may also vary spatially and temporally. Furthermore, assessment of trace metals mainly relies on in situ observation, and so wide-area investigation of trace-metal concentration may be challenging and subject to technical constraints. A specific approach is therefore necessary that combines biogeochemical proxies, satellite data, and trace-metal linear correlation. This study aims to assess the potential spatio-temporal variability of sea surface cadmium (Cd) and copper (Cu) concentrations in Indonesian seas and surrounding areas. The correlations of Cd and Cu concentrations with primary production and nutrient data were used to convert hindcast satellite data into estimates of the metals' concentrations. The potential variability of trace metals can be determined by overlaying both data. Indonesia's Fisheries Management Areas (FMAs) were used for data clustering and analysis. The results show that Cd and Cu trace metals have similar distribution patterns throughout the year. However, dissolved Cu has a more diverse coverage area than dissolved Cd, including within the Halmahera, Seram, and Maluku Seas (FMAs 716 and 717), the Makassar Strait (FMA 717), and the Java-Sumatra upwelling area (FMA 573). Both Cd and Cu concentrations in the Java-Sumatra upwelling region follow the periodic upwelling pattern. Overall, both Cd and Cu show a declining trend in concentration from 2012 to 2019. It is estimated that dissolved Cd concentration declined from 1500-2000pmol/kg in 2012 to 1000-1500pmol/kg in 2019 for all locations. Dissolved Cu concentration decreased from 30-35nmol/kg in 2012 to 25-30nmol/kg in 2019. Estimated dissolved Cd and Cu follow the linear functions of silicate (SiO4), nitrate (NO3), and primary productivity. The fluctuation of anthropogenic activities and global warming are likely to indirectly impact the decline in metal concentrations by affecting nutrients and primary productivity.

Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

Linking variability in early life growth of Sardinella lemuru to changes in habitat conditions in Zamboanga upwelling system, Sulu Sea, the Philippines

The year-class success of small pelagic fish is strongly modulated by the growth and survival of early life stages. Determining how environments control growth is critical to understanding processes influencing annual production. We provide the first measurements of larval growth in Sardinella lemuru, the most abundant and exploited tropical sardine species in the Philippines. Growth rates of larvae were examined across a 2 yr period (2011-2012 and 2012-2013 spawning seasons) using otolith microstructure analysis. Transition to the juvenile stage occurred within 25 to 37 d. Mean growth rate of the 2011 year-class was significantly lower than the 2012 year-class despite significant intra-annual (batch) differences in growth trajectories. In both years, larvae that hatched and developed in the cooler yet prey-rich period in the middle of the spawning season (January) displayed the fastest growth, while those spawned early (September-October 2012) and late (February-March 2012) showed the slowest growth rates. Peak hatch-months (December 2011 and October 2012) were just prior to periods of upwelling, with mean prey concentrations of 0.24 ± 0.13 mg m-3 at 26.35 ± 2.56°C sea surface temperature (SST) and 0.23 ± 0.15 mg m-3 at 26.26 ± 1.79°C, respectively. The SST during upwelling was 0.01 to 1.23°C cooler accompanied by a 0.07 to 0.45 mg m-3 increase in prey concentration. In both years, differences in growth rate were strongly linked to oceanographic conditions. Moderate winds during a ‘neutral’ El Niño-Southern Oscillation year resulted in upwelling conditions favoring faster overall growth in larvae which was associated with stronger recruitment. In contrast, slower growth and weaker recruitment were associated with weak upwelling conditions in the 2011-2012 La Niña year. While temperature may be important, larval growth rates appear to be driven more by prey abundance.

Glider Observations of the Northwestern Iberian Margin During an Exceptional Summer Upwelling Season

AbstractGlider observations from the Northwestern Iberian Margin during the exceptionally strong 2010 summer upwelling season resolved the evolution of physical and biogeochemical variables during two upwelling events. Upwelling brought low‐oxygen Eastern North Atlantic Central Water from 190 m depth onto the shelf up to a depth of 50 m. During the two observed periods of upwelling, a poleward jet developed over the shelf break. The persistent upwelling favorable winds maintained equatorward flow on the outer shelf for 2 months with no reversals during relaxation periods, a phenomenon not previously observed. During upwelling, near‐surface chlorophyll a concentration increased by more than 6 mg m−3. Oxygen supersaturation in the near surface increased by more than 20%, 6 days after the chlorophyll a maximum.

Long-term impact of Central Basin hypoxia and internal phosphorus loading on north shore water quality in Lake Erie

ABSTRACTWe hypothesized that north shore water quality of Lake Erie’s Central Basin is impacted by Central Basin hypoxia and, by extension, related to redox-dependant internal phosphorus (P) loading from the profundal sediments. To evaluate this hypothesis, we first quantified Central Basin hypoxia as hypoxic factor (HF, days in Aug–Sep that a sediment area equal to the Central Basin surface area was hypoxic) from published annual hypoxic (<2.0 mg/L dissolved oxygen) areal extent and duration. From 1985 to 2012, mean HF values were 15.2 (SE 1.45) d/Aug–Sep and ranged between 0 (1996) and 34.3 (2012) d/Aug–Sep. Second, we estimated internal load from an areal P release rate of 8 mg/m2/d, multiplied by HF. The estimate of 122 (SE 11.6) mg/m2/Aug–Sep (1985–2012, n = 28) is supported by an independent estimate of 136 (SE 20.3) mg/m2/summer (1970–1986, n = 15) computed from total P concentration increases at fall turnover and by an estimate for 1970. Third, 2013 temperature and oxygen profiles demonstrated periodic upwelling on the north shore of the Central Basin, in agreement with other studies. Fourth, we analysed P, chlorophyll a (Chl-a) and phytoplankton species data at 2 north shore drinking water intake stations. HF and internal loading were significantly correlated with summer soluble reactive P, August–October Chl-a concentrations, and cyanobacteria abundance at the Central Basin site but not at the control site in the Western Basin. While these correlations are weak, suggesting other effects including benthic productivity, they may be partially caused by hypolimnetic water circulated toward the shoreline.

A Late Quaternary climate record based on long-chain diol proxies from the Chilean margin

Abstract. In this study we have applied different indices based on long-chain diols, i.e., the long-chain diol index (LDI) as a proxy for past SST, the diol index as an indicator of past upwelling conditions, and the nutrient diol index (NDI) as a proxy for nitrate and phosphate concentrations in seawater. The proxies were analyzed in marine sediments recovered at ODP Site 1234, located within the Peru–Chile upwelling system, with a ∼2 kyr resolution covering the last 150 kyr. We also generated TEX86H and U37K′ temperature and planktonic δ18O records, as well as total organic carbon (TOC) and accumulation rates (ARs) of TOC and lipid biomarkers (i.e., C37 alkenones, GDGTs, dinosterol, and loliolide) to reconstruct past phytoplankton production. The LDI-derived SST record covaries with TEX86H- and U37K′-derived SST records as well as with the planktonic δ18O record, implying that the LDI reflects past SST variations at this site. TOC and phytoplankton AR records indicate increased export production during the last interglacial (MIS 5), simultaneous with a peak in the abundance of preserved Chaetoceros diatoms, suggesting intensified upwelling during this period. The diol index is relatively low during the upwelling period, but peaks before and after this period, suggesting that Proboscia diatoms were more abundant before and after the period of upwelling. The NDI reveals the same trends as the diol index, suggesting that the input of nitrate and phosphate was minimal during upwelling, which is unrealistic. We suggest that the diol index and NDI should perhaps be considered as indicators for Proboscia productivity instead of upwelling conditions or nutrient concentrations.

Temporal variability of downward fluxes of organic carbon off Monterey Bay

Sediment traps were deployed at two depths (300 m and 1200 m) off Monterey Bay (36°40′N and 122°25′W, Central California) for 7.3 years (1998–2005). The sediment trap data provided information about the quantity and quality of settling material, and allowed exploration of the relationship of the sinking material with the environmental conditions in this coastal upwelling region. The magnitude and composition of the settling material were highly variable over time. Organic carbon (Corg) fluxes ranged between 4–296 mg C m−2 day−1 and 0.1–142 mg C m−2 day−1 for shallow and deep sediment traps, respectively. The time series of Corg vertical flux was characterized by pulses of intense fluxes that were associated with peaks of primary production, generally during upwelling periods. Despite considerable variability, fluxes varied seasonally with highest values during the upwelling season and the lowest in winter. Attenuation of Corg vertical fluxes with depth (300 m vs. 1200 m) varied between 31% and 24% except for the late upwelling period, when there was an increase with depth likely due to resuspension of material from Monterey Canyon. Calculation of a seasonal vertical budget of organic carbon off Monterey Bay resulted in a transfer between 4.0% and 4.9% of the primary production to the deep ocean, suggesting that coastal upwelling efficiently sequestered CO2.

Presence, Behavior, and Resighting Pattern of Transient Bottlenose Dolphins (Tursiops truncatus) in the Humboldt Current System off North-Central Chile

Biotic and abiotic factors determine presence and habitat use pattern of individuals within a population. In this study, presence, behavior, and resighting patterns of transient bottlenose dolphins (Tursiops truncatus) were evaluated in relation to upwelling and downwelling events in a marine reserve in North-Central Chile, between 2005 and 2009. The study period was divided into four phases according to wind direction and intensity: upwelling-favorable (UF), transition I (TI), convergence (Cv) or downwelling, and transition II (TII). Results show that transient bottlenose dolphins are an open population with low resighting rates. Highest occurrence and a largest number of transient dolphins were identified during 2009, probably due to an increase in prey availability. The most frequent behavior observed was traveling, followed by feeding and socializing. Traveling was mainly recorded in individuals seen only once and in years with low productivity. In contrast, feeding was observed in individuals seen two or more times, was similar among phases, and was more frequent in moreproductive years. Social behavior was associated with the highest resighting rates. This study documents how transient bottlenose dolphins use the area based on their resighting patterns and suggests that periods of upwelling and downwelling modulate behavior displayed by these dolphins within the area.

Wavelet analysis on upwelling index along the Moroccan Atlantic coast

Wavelet analysis is a relatively new technique, an enormous interest in the application of wavelets has been observed in the recent years. In this paper, we use the wavelet analysis on the upwelling index based on sea surface temperature data from 1993 to 2014 along the Moroccan Atlantic coast, mainly at Cape Cantin, Cape Ghir, Cape Juby, Dakhla and Cape Blanc. We noticed that from Jan 1993 to Oct-1998 and from Sep-2007 to Nov 20014, the upwelling in the five research latitudes increases with fluctuations. The period of Upwelling varies sensitively with the change of time scale. The value of upwelling index in the five latitudes appears similar and present an obvious period of one year in all latitudes studied. This study can be considered an important tool for time series analysis, which can help the studies concerning the upwelling variability.

Growth and survival of the winged oyster Pteria colymbus in suspended culture: influence of environmental factors associated to upwelling periods

The present study examines the influence that environmental variables exerted on changes in condition index (CI), shell height (SH—dorsal-ventral axis) and soft tissue mass increments (STM) of the winged oyster Pteria colymbus in suspended culture during periods of upwelling (December to April) and non-upwelling (August to November) in the Cariaco Gulf, northeastern Venezuela. Environmental variables recorded between April 2012 and May 2013 included seston, water transparency, temperature, dissolved oxygen, chlorophyll a (Chl a) and the upwelling index (UI). Individuals were cultivated in lantern nets with an overall density of 56 individuals/0.123 m2. From three randomly chosen baskets, five individuals were sampled each month and changes in CI, STM and SH were related to the environmental variables using Spearman correlation and PCA. Results show that during upwelling, both Chl a and UI presented a positive and significant relationship with the oyster condition and growth parameters. During non-upwelling (low UI), this relationship was inverse, demonstrating the important influence of upwelling and non-upwelling periods on the ecophysiology of the species. Furthermore, it was shown that the non-upwelling, characterized by high temperatures and low food availability, is a critical period for P. colymbus, judging by a decrease in growth and survival rate. Nonetheless, this species reached 50 mm in 5 months, a size considered as commercially viable, suggesting that the study area is favourable to the cultivation of the winged oyster despite a dramatic decrease in upwelling intensity in the last decade.

Linking spatial distribution and feeding behavior of Atlantic horse mackerel (Trachurus trachurus) in the Strait of Sicily (Central Mediterranean Sea)

The Atlantic horse mackerel (Trachurus trachurus) is a semi-pelagic fish species with a wide geographical distribution and commonly found on the continental shelf. In recent years, the species has received some attention due to its ecological role in pelagic food webs. Stable isotope of nitrogen and carbon (δ15N and δ13C) and Stomach Content Analysis (SCA) of T. trachurus were adopted as tools to provide necessary knowledge on its feeding habits in the Strait of Sicily. The strong correlation found between δ15N and δ13C values with body length, together with significant ontogenetic shift evidenced by SCA, could be associated to changes in food availability, which in turn is triggered by environmental conditions. Spatial distribution of T. trachurus in the study area, mainly for small and medium size specimens, is linked to lower temperature, salinity and Photosynthetically Active Radiation (PARsat) values with higher aggregations on the Adventure Bank (i.e., an area characterized by periodic upwelling events). Larger size specimens mostly inhabited shallower waters characterized by higher temperature, salinity and PAR values, typical of the central-eastern part of the study area. Our results support the hypothesis that feeding behavior of T. trachurus is strictly linked to environmental factors (i.e. chiefly oceanographic conditions of the water column and day duration) that in turn influences the distribution of its prey. Findings can supply knowledge needed for improving fish stock management and promoting plans able to take into account also local ecosystem analysis.

Bryde’s whales (Balaenoptera brydei) in an area of upwelling off Isla San Cristóbal, Galápagos

Bryde’s whales aggregate in the Galápagos Archipelago in an area off the north coast of Isla San Cristóbal during periods of upwelling. Seasonally, these whales are locally abundant off San Cristóbal between May and November, except in years when El Niño conditions occur. Field surveys were performed in summers 2013, 2014, and 2016 around San Cristóbal to search for the presence of Bryde’s whales, and to document their spatial relationship with 20 °C isotherm topography. Our surveys found that most sightings of these whales occurred in locations within 3–6 km of shore (water depths 70–300 m), where the 20 °C isotherm depth shoaled to an average of 31 m below the surface. The presence of Blue whales (Balaenoptera musculus), as well as Bryde’s whales was documented during 2016 upwelling. Mother-calf pairs of Bryde’s and Humpback whales were also documented off San Cristóbal in summers 2013 and 2016 and 2014, respectively. We suggest that our three summers of sightings of Bryde’s whales in an area of upwelling indicates that Isla San Cristóbal is important habitat for individuals and mother/calf pairs alike.

Identifying the Sources and Sinks of CDOM/FDOM across the Mauritanian Shelf and Their Potential Role in the Decomposition of Superoxide (O2-)

Superoxide (O2-) is a short lived reactive oxygen species (ROS) formed in seawater by photochemical or biological sources, it is important in the redox cycling of trace elements and organic matter in the ocean. The photoproduction of O2- is now thought to involve reactions between O2 and reactive reducing (radical) intermediates formed from dissolved organic matter (DOM) via intramolecular reactions between excited singlet state donors and ground-state acceptors (Zhang et al., 2012). In seawater the main pathways identified for the decomposition of O2- into H2O2 and O2, involve reactions with Cu, Mn and DOM. In productive regions of the ocean, the reaction between DOM and O2- can be a significant sink for O2-. Thus DOM is a key component of both the formation and decomposition of O2- and formation of H2O2. In the present work we examined the relationships between O2- decay rates and parameters associated with chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) by using the thermal O2- source SOTS-1. Filtered samples (0.2 µm) were run both in the presence, and absence, of the metal chelator diethylenetriaminepentaacetic acid (DTPA) to determine the contribution from DOM. Samples were collected along a transect across the continental shelf of the Mauritanian continental shelf during a period of upwelling. In this region we found that reactions with DOM, are a significant sink for O2- in the Mauritanian Upwelling, constituting on average 58 ± 13 % of the O2- loss rates. Superoxide reactivity with organic matter showed no clear correlation with bulk CDOM or FDOM properties (as assessed by PARAFAC analysis) suggesting that future work should concentrate at the functional group level to clearly elucidate which molecular species are involved as bulk properties represent a wide spread of chemical moieties with different O2- reactivities. Analysis of FDOM parameters indicates that many of the markers used previously for terrestrial sources of DOM and FDOM are called into question as marine sources exist. In particular recent work (Rico et al., 2013) indicates that algal species may also produce syringic, vanillic and cinnamic acids, which had previously been ascribed solely to terrestrial vegetation.

A seasonal comparison of prokaryote numbers, biomass and heterotrophic productivity in waters of the KwaZulu-Natal Bight, South Africa

The KwaZulu-Natal Bight is a shallow indentation of the eastern seaboard of South Africa, characterised by a narrow (45 km wide) extension of the continental shelf, with a shelf break at about 100 m. It has a complex hydrography: the waters of the bight are derived from the fast-flowing, southward-trending Agulhas Current, which is fed mostly by the tropical and subtropical surface waters of the South-West Indian Ocean subgyre, which are generally oligotrophic in nature, notably depleted in reduced nitrogen and phosphate except at river mouths and during periodic upwelling of deeper nutrient-rich water. Despite this, the bight is believed to be relatively productive, and it is suggested that efficient nutrient recycling by prokaryotes may sustain primary productivity efficiently, even in the absence of new nutrient inputs. Here we have measured bacterial numbers, biomass and heterotrophic productivity during summer and winter in conjunction with phytoplankton standing stock and factors that influence it. Bacterial distribution closely matched phytoplankton distribution in surface waters, and was highest close to the coast. Bacterial standing stocks were similar to those of oligotrophic systems elsewhere (0.5–5.0 × 105 cells ml–1; 1 × 10–8 to 1.25 × 10–7 g C ml–1) and increased in association with the development of phytoplankton blooms offshore and with inputs of allochthonous material by rivers at the coast. Heterotrophic productivity in summer was lowest in the far south and north of the bight (0.5 × 10–10 g C ml–1 h–1) but higher close to the shore, over shallow banks, and in association with increased phytoplankton abundance over the midshelf (1.0–3.5 × 10–9 g C ml–1 h–1). There were marked seasonal differences with lower bacterial standing stocks (5 × 104 to 2 × 105 cells ml–1; 4–5 × 10–9 to 1–2 × 10–8 g C ml–1) and very low bacterial productivity (4 × 10–11 to 1 × 10–10 g C ml–1 h–1) in winter, probably resulting from lowered rates of primary productivity and dissolved organic matter release as well as reduced riverine allochthonous inputs during the winter drought.

Morphometric and stable isotopic differentiation of Orbulina universa morphotypes from the Cariaco Basin, Venezuela

Abstract Biometric characteristics of Orbulina universa (d'Orbigny) were used to differentiate two morphotypes present in sediment trap samples collected from the Cariaco Basin, Venezuela. Specifically, wall thickness and weight–area relationships were used to separate shells into thin (Mthin) and thick (Mthick) morphotypes. Mthick (mean thickness = 19–41 μm) comprises 75% of the total O. universa in these samples and has morphometric characteristics similar to that of the previously described Type I Caribbean genotype, whereas Mthin (mean thickness = 6–22 μm) is comparable to the Type III Mediterranean genotype. The flux of Mthick increases during periods of upwelling, whereas Mthin flux shows no systematic relationship with changing hydrographic regimes in the basin. The δ18O and δ13C of Mthick are on average 0.34‰ higher and 0.38‰ lower, respectively, than those of Mthin, suggesting that they calcify their final spherical chamber at different depths in the water column and/or differ in their vital effects on shell geochemistry. Additionally, the absolute offset in the stable isotopic compositions of the two morphotypes varies as a function of surface ocean stratification. During periods of upwelling, the δ18O and δ13C absolute offsets between Mthin and Mthick are on average − 0.15 and 0.25‰, whereas average δ18O and δ13C absolute offsets between these two morphotypes increase to − 0.54 and 0.41‰ during non-upwelling periods. We attribute the higher δ18O and δ13C offsets during non-upwelling periods to a difference in final chamber calcification depth and the combined effects of temperature (δ18O), δ13CDIC (δ13C), irradiance (δ13C) and [CO32 −] (δ18O and δ13C). These data provide field evidence that thin and thick morphotypes of O. universa likely experience different environmental conditions during the formation of their final chamber and, therefore, should not be combined in geochemical analyses for reconstructing past surface ocean conditions. Finally, we introduce a new proxy for reconstructing past surface ocean stratification changes via the use of Mthin and Mthick δ18O and δ13C offsets.