Antonio Bode

Large mesopelagic fishes biomass and trophic efficiency in the open ocean

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters.

A persistent upwelling off the Central Cantabrian Coast (Bay of Biscay)

Abstract North Atlantic Central Waters were detected near the surface in some locations off the Central Cantabrian Coast (Bay of Biscay). This feature suggests an upwelling process taking place in the season of the thermal stratification which is persistent throughout the summer. The exact mechanism of formation is unknown but the wind driven currents are probably the main force. The prevailing winds off the Cantabrian coast are northeast during summer, generating westward superficial currents that produce an Eckman transport offshore. Factors such as shape of the coast and slope topography maybe modulators. Although the upwelling was subsuperficial, our results suggest that cold and nutrient rich waters reached the surface previously. The biomass distribution of phytoplankton varied according to the upwelling and different populations could be recognized. In stations not affected by upwelling phytoplankton occured mainly on top of the thermocline, whereas in those affected by upwelled waters it was concentrated near the surface. The mean size and composition of the populations were also different, and two distinct types could be recognized and related to nutrient concentrations.

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

SummaryChlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m−2 and 86.9 ± 44.0 mg C m−2 h−1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower.

Preliminary Studies on the Export of Organic Matter During Phytoplankton Blooms off La Coruña (Northwestern Spain)

Sinking of particles was measured at a coastal site near La Coruna (north-western Spain) using sedimentation traps placed at the base of the euphotic zone during four short periods between March and June 1995. In addition, measurements of water column properties and particles were made at five stations. The cases studied were representative of the main oceanographic situations that occur in this area and were related to the windinduced upwelling. Phytoplankton blooms ( > 100 mg chlorophyll-a m-‘) were observed on two occasions in April and June, both during favourable upwelling conditions. The two other cases were studied during low phytoplankton biomass concentrations in the water column. The mean values of sinking carbon varied between 1203 and 323 mg C me2 d-‘. The minimum values were measured during the upwelling case in June, and may be explained in part by the offshore displacement of surface water caused by the upwelling. The contribution of phytoplankton to the exported carbon was low (1045% of the total particulate carbon), and only a small fraction corresponded to Living phytoplankton (24%). Most of the particles collected in the traps appeared to be of pelagic origin, mainly because low C:N ratios, dominance of phytoplanktonic over phytobenthic species and the abundance of zooplankton faecal pellets. The trap-collected material displayed a sample of the phytoplankton species present, but some species appeared with higher frequencies than expected from their abundances in the water column. In addition, the sedimented material showed a high abundance of faecal pellets. The abundance of the smaller faecal pellets’( 200 pm) were more abundant in May, matching the variation in the abundance of herbivorous and omnivorous copepods in the water column.

Preliminary studies on the reproduction and population dynamics of Monodonta lineata and Gibbula umbilicalis (Mollusca, Gastropoda) on the central coast of Asturias (N. Spain)

SummaryPopulations of Monodonta lineata and Gibbula umbilicalis were studied at Aramar beach (Luanco, Asturias) from October 1982 to November 1983. Both species show a sexual ratio of 1: 1 but differ in their gonadal cycle patterns. M. lineata gonadal development stages are found from November to May–June in a significant part of the population. Breeding stages occur from June–July to September, and in some individuals to November, with complete discharge of all spawn at the end of the reproductive season. Some degree of inter-annual variation is detected. Spawning stages of mature G. umbilicalis are found through the year but the higher frequencies occur in summer and autumn samples, with development phases predominating in January to May. Animals over 10 mm in shell diameter maintain their gonads close to spawning stages, completely spawned individuals being less than 10 percent. In both species variability in gonad stages for a given month reduces as size increases, indicating a certain synchrony.The new recruits of M. lineata are detected on the bare rock by September to November and are recognizable through their first year. Due to the amplitude of the breeding period and differing growth rates size distribution within an age class is wide. Size class frequencies rapidly decrease after the second year of life. The data for G. umbilicalis are difficult to interpret given the scarce number of juveniles and size overlapping.

Importance of salt fingering for new nitrogen supply in the oligotrophic ocean.

The input of new nitrogen into the euphotic zone constrains the export of organic carbon to the deep ocean and thereby the biologically mediated long-term CO2 exchange between the ocean and atmosphere. In low-latitude open-ocean regions, turbulence-driven nitrate diffusion from the oceans interior and biological fixation of atmospheric N2 are the main sources of new nitrogen for phytoplankton productivity. With measurements across the tropical and subtropical Atlantic, Pacific and Indian oceans, we show that nitrate diffusion (171±190 μmol m−2 d−1) dominates over N2 fixation (9.0±9.4 μmol m−2 d−1) at the time of sampling. Nitrate diffusion mediated by salt fingers is responsible for ca. 20% of the new nitrogen supply in several provinces of the Atlantic and Indian Oceans. Our results indicate that salt finger diffusion should be considered in present and future ocean nitrogen budgets, as it could supply globally 0.23–1.00 Tmol N yr−1 to the euphotic zone.

Dispersal similarly shapes both population genetics and community patterns in the marine realm

Dispersal plays a key role to connect populations and, if limited, is one of the main processes to maintain and generate regional biodiversity. According to neutral theories of molecular evolution and biodiversity, dispersal limitation of propagules and population stochasticity are integral to shaping both genetic and community structure. We conducted a parallel analysis of biological connectivity at genetic and community levels in marine groups with different dispersal traits. We compiled large data sets of population genetic structure (98 benthic macroinvertebrate and 35 planktonic species) and biogeographic data (2193 benthic macroinvertebrate and 734 planktonic species). We estimated dispersal distances from population genetic data (i.e., FST vs. geographic distance) and from β-diversity at the community level. Dispersal distances ranked the biological groups in the same order at both genetic and community levels, as predicted by organism dispersal ability and seascape connectivity: macrozoobenthic species without dispersing larvae, followed by macrozoobenthic species with dispersing larvae and plankton (phyto- and zooplankton). This ranking order is associated with constraints to the movement of macrozoobenthos within the seabed compared with the pelagic habitat. We showed that dispersal limitation similarly determines the connectivity degree of communities and populations, supporting the predictions of neutral theories in marine biodiversity patterns.

Fate of organic matter in the Ría de Ferrol (Galicia, NW Spain): uptake by pelagic bacteria vs. particle sedimentation

Abstract A seasonal study was conducted in the Ria de Ferrol (Galicia, NW Spain) to determine the relative importance of bacterial removal of dissolved organic matter and sedimentation of particulate organic matter as loss processes of organic matter in the water column. Four seasonal cruises covered three different zones of the ria. Thermohaline stratification induced by continental runoff was evident during all the cruises, especially in the inner part of the ria, while upwelling of shelf water only affected the water layer below 15 m in the outer ria during summer. The abundance of heterotrophic bacteria ranged from 0.3 × 106 cell ml–1 in February up to 1.8 × 106 cell ml–1 in July. The lowest rate of bacterial production was measured in February (5.3 pM Leu h–1) and the highest value was measured in July (527.5 pM Leu h–1). Generally, both bacterial abundance and production displayed maximum values at subsurface layers and in the inner part of the ria. Dissolved organic carbon (DOC) and particulate organic carbon (POC) ranged from 7.4 to 31.7 and from 1.3 to 8.9 g C m–2, respectively, and both the carbon sources increased from February to September. In contrast, maximum values of the sedimented particulate carbon (SPC) were measured in February, when bacterial production was low (5.8–9.6 g C m–2 d–1), while minimum values of SPC were detected in September (1.2–3.2 g C m–2 d–1). Carbon biomass and production of pelagic bacteria in the Ria de Ferrol were coupled to increases in both temperature and chlorophyll-a. As a consequence, the flux of DOC through pelagic bacteria increased from winter to summer. However, bacteria removed less than 10% of DOC stock, and particle sedimentation was the main pathway in the loss of organic matter from the water column.

Variability in δ15N of intertidal brown algae along a salinity gradient: Differential impact of nitrogen sources.

While it is generally agreed that δ(15)N of brown macroalgae can discriminate between anthropogenic and natural sources of nitrogen, this study provides new insights on net fractionation processes occurring in some of these species. The contribution of continental and marine sources of nitrogen to benthic macroalgae in the estuary-ria system of A Coruña (NW Spain) was investigated by analyzing the temporal (at a monthly and annual basis) and spatial (up to 10 km) variability of δ(15)N in the macroalgae Ascophyllum nodosum and three species of the genus Fucus (F. serratus, F. spiralis and F. vesiculosus). Total nitrate and ammonium concentrations and δ(15)N-DIN, along with salinity and temperature in seawater were also studied to address the sources of such variability. Macroalgal δ(15)N and nutrient concentrations decreased from estuarine to marine waters, suggesting larger dominance of anthropogenic nitrogen sources in the estuary. However, δ(15)N values of macroalgae were generally higher than those of ambient nitrogen at all temporal and spatial scales considered. This suggests that the isotopic composition of these macroalgae is strongly affected by fractionation during uptake, assimilation or release of nitrogen. The absence of correlation between macroalgal and water samples suggests that the δ(15)N of the species considered cannot be used for monitoring short-term changes. But their long lifespan and slow turnover rates make them suitable to determine the impact of the different nitrogen sources integrated over long-time periods.

Seasonal variations in upwelling and in the grazing impact of copepods on phytoplankton off A Coruña (Galicia, NW Spain)

Abstract The impact of grazing by copepods on phytoplankton was studied during a seasonal cycle on the Galician shelf off A Coruna (NW Spain). Grazing was estimated by measuring the chlorophyll gut content and the evacuation rates of copepods from three mesh-size classes: 200–500 (small), 500–1000 (medium), and 1000–2000 μm (large). Between February 1996 and June 1997, monthly measurements of water temperature, chlorophyll concentration, primary production rates, and copepod abundance, chlorophyll gut content, and evacuation rates were taken at an 80-m-deep, fixed shelf station. Additionally, the same measurements were collected daily during two bloom events in March and in July 1996. Small copepods were the most abundant through the seasonal cycle. The highest grazing impact, however, was due to the medium and large size classes. Grazing by small copepods exceeded grazing by medium and large copepods only during phytoplankton spring blooms. The impact of copepod grazing (considering all size fractions) was generally low. On average, 2% of the phytoplankton biomass and 6% of the primary production were removed daily by the copepod community. Maximum grazing impact values (9% of the phytoplankton biomass and 39% of the primary production) were found in mid-summer. These results suggest that most of the phytoplankton biomass would escape direct copepod grazing in this upwelling area.