Benthic O2 availability regulates many important biogeochemical processes and has crucial implications for the biology and ecology of benthic communities. Further, the benthic O2 exchange rate represents the most widely used proxy for quantifying mineralization and primary production of marine sediments. Consequently, numerous researchers have investigated the benthic O2 dynamics in a wide range of environments. On the basis of case studies from abyssal sediments to microbial phototrophic communities I hereby try to review the current status on what we know about controls that interrelate with the O2 dynamics of marine sediments. This includes factors like: sedimentation rates, bottom water O2 concentrations, diffusive boundary layers, fauna activity, light, temperature, and sediment permeability. The investigation of benthic O2 dynamics represents a challenge in resolving variations on temporal and spatial scales covering several orders of magnitude. Such an effort requires the use of several complementary measuring techniques and modeling approaches. Recent technical developments (improved chamber approaches, O2 optodes, eddy-correlation, benthic observatories) and advances in diagenetic modeling have facilitated our abilities to resolve and interpret benthic O2 dynamics. However, all approaches have limitations and caveats that must be carefully evaluated during data interpretation. Much has been learned during the last decades but there are still many unanswered questions that need to be addressed in order to fully understand benthic O2 dynamics and the role of sediments for marine carbon cycling.

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Oxygen dynamics of marine sediments

Publisher Summary This chapter explores that the pelagic food chain is mainly linear and short, and there is a relatively close coupling between the primary production and the production of (pelagic) fish in the oceans. It has been realized that pico- and nano-sized phytoplankton (e.g. cyanobacteria) and heterotrophic micro-organisms (heterotrophic bacteria, heterotrophic nanoflagellates and ciliates) play a much larger quantitative role in production and mineralization, respectively, of the phytoplankton than formerly believed. Moreover, as microbial food webs are typically long and primarily based upon regenerated phytoplankton production, microbial production contributes insignificantly to fish production in the oceans. This chapter illustrates the significance of net-phytoplankton blooms to the fisheries production in the ocean, first of all blooms associated with larger-scale physical processes, such as the major upwelling regions and the vernal temperature stratification in temperate waters.

Turbulence, Phytoplankton Cell Size, and the Structure of Pelagic Food Webs

The Humboldt Current System (HCS) is one of the most productive marine ecosystems on earth. It extends along the west coast of South America from southern Chile (∼42°S) up to Ecuador and the Galapagos Islands near the equator. The general oceanography of the HCS is characterised by a predominant northward flow of surface waters of subantarctic origin and by strong upwelling of cool nutrient-rich subsurface waters of equatorial origin. Along the coast of northern and central Chile, upwelling is localised and its occurrence changes from being mostly continuous (aseasonal) in northern Chile to a more seasonal pattern in southern-central Chile. Several important upwelling centres along the Chilean coast are interspersed with long stretches of coast without or with sporadic and less intense upwelling. Large-scale climatic phenomena (El Nino Southern Oscillation, ENSO) are superimposed onto this regional pattern, which results in a high spatiotemporal heterogeneity, complicating the prediction of ecological processes along the Chilean coast. This limited predictability becomes particularly critical in light of increasing human activities during the past decades, at present mainly in the form of exploitation of renewable resources (fish, invertebrates and macroalgae). This review examines current knowledge of ecological processes in the HCS of northern and central Chile, with a particular focus on oceanographic factors and the influence of human activities, and further suggests conservation strategies for this high-priority large marine ecosystem. Along the Chilean coast, the injection of nutrients into surface waters through upwelling events results in extremely high primary production. This fuels zooplankton and fish production over extensive areas, which also supports higher trophic levels, including large populations of seabirds and marine mammals. Pelagic fisheries, typically concentrated near main upwelling centres (20-22°S, 32-34°S, 36-38°S), take an important share of the fish production, thereby affecting trophic interactions in the HCS. Interestingly, El Nino (EN) events in northern Chile do not appear to cause a dramatic decline in primary or zooplankton production but rather a shift in species composition, which affects trophic efficiency of and interactions among higher-level consumers. The low oxygen concentrations in subsurface waters of the HCS (oxygen-minimum zone, OMZ) influence predator-prey interactions in the plankton by preventing some species from migrating to deeper waters. The OMZ also has a strong effect on the bathymetric distribution of sublittoral soft-bottom communities along the Chilean coast. The few long-term studies available from sublittoral soft-bottom communities in northern and central Chile suggest that temporal dynamics in abundance and community composition are driven by interannual phenomena (EN and the extent and intensity of the OMZ) rather than by intra-annual (seasonal) patterns. Macrobenthic communities within the OMZ are often dominated in biomass by sulphide-oxidising, mat-forming bacteria. Though the contribution of these microbial communities to the total primary production of the system and their function in structuring OMZ communities is still scarcely known, they presumably play a key role, also in sustaining large populations of economically valuable crustaceans.

The humboldt current system of northern and central chile : Oceanographic processes, ecological interactions and socioeconomic feedback

The high biological productivity of the Humboldt Current System (HCS) off Chile supports an annual fish catch of over 7 million t. The area is also important biogeochemically, because the outgassing of recently upwelled water is modulated by contrasting degrees of biological activity. However, very few field measurements of primary production and planktonic respiration have been undertaken within the Eastern Boundary Current (EBC) system off Chile. In this study an estimate of primary production (PP) and surface planktonic community respiration is presented from several research cruises in the HCS and adjacent oceanic areas. The highest production levels were found near the coast correlating closely with known upwelling areas. Both gross primary production (GPP) and community respiration (CR) showed important spatial and temporal fluctuations. The highest water column integrated GPP was measured in the southern and central fishing area (19.9 g C m -2 d -1 ) and off the Antofagasta upwelling ecosystem (9.3 g C m -2 d -1 ). The range of GPP agrees well with values reported for Peru (0.05 to 11.7 g C m -2 d -1 ). The Coquimbo upwelling system, despite being an area of persistent upwelling, showed lower production and community respiration values than the Antofagasta and Concepcion Shelf areas.The lowest surface PP values were measured within the oceanic region adjacent to the northern coastal upwelling zones (0.8 ± 0.5 μg C l -1 h -1 ), though slightly enhanced biological production was found within the Nazca Ridge (1.5 ± 2.1 μg C l -1 h -1 ) that separates the Chilean and Peruvian deep basins.

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Primary production and community respiration in the Humboldt Current System off Chile and associated oceanic areas

Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Conception Bay at 36.5”s. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in l-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (0,, N03-, and oxides of Mn and Fe) and from directly determined rates of SOd2- reduction. The study area is characterized by intense seasonal upwelling, and during sampling in late summer the bottom water over the shelf was rich in NO,- and depleted of OZ. Sediments at the four shelf stations were covered by mats of filamentous bacteria of the genera Thioploca and Beggiatoa. Carbon oxidation rates at these sites were extremely high near the sediment surface (> 3 pmol cmm3 d-l) and decreased exponentially with depth. The process was entirely coupled to SOd2reduction. At the two slope stations where bottom-water O2 was > 100 PM, C oxidation rates were lo-fold lower and varied less with depth; C oxidation coupled to the reduction of 02, N03-, and Mn oxides combined to yield an estimated 15% of the total C oxidation between 0 and 10 cm. Carbon oxidation through Fe . reduction contributed a further 12-29% of the depth-integrated rate, while the remainder of C oxidation was through SOd2- reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool decreased with depth, the importance of SOd2- reduction increased. The results point to a general importance of Fe reduction in C oxidation in continental margin sediments. At the shelf stations, Fe reduction was mainly coupled to oxidation of reduced S. These sediments were generally H,S-free despite high SOd2- reduction rates, and precipitation of Fe sulfides dominated H,S scavenging during the incubations. A large NO,- pool was associated with the Thioploca, and the shelf sediments were thus enriched in N03- relative to the bottom water, with maximum concentrations of 3 pmol cm-3. The NO,- was consumed during our sediment incubations, but no effects on either C or S cycles could be discerned.

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Pathways of carbon oxidation in continental margin sediments off central Chile.

The nearshore zone during coastal upwelling: Daily variability and coupling between primary and secondary production off central Chile

The jack mackerel fishery and El Niño 1997–98 effects off Chile

Seasonal growth of small pelagic fish off Talcahuano, Chile (37°S, 73°W): a consequence of their reproductive strategy to seasonal upwelling?

Recruitment of common sardine (Strangomera bentincki) and anchovy (Engraulis ringens) off central-south Chile in the 1990s and the impact of the 1997-1998 El Niño

The influence of upwelling on the distribution of chlorophyll a within the Bay of Concepcion, Chile is discussed in light of continuous measurements of surface in vivo chlorophyll fluorescence and temperature taken simultaneously along horizontal transects, and hydrographic stations' data. Results suggest significant temporal variability both in the distribution of in vivo fluorescence, temperature and salinity within the Bay and in the characteristics of the exchange between the Bay and the adjacent shelf waters, induced by variable upwelling. Upwelling is produced by the predominant south-westerly winds during the summer. Significant variations in the wind direction occur with periods from two to seven days. During active upwelling, exchange is characterized by a surface outflow through the mouth of the Bay and an inflow at depth. Low chlorophyll fluorescence is confined to the upwelling areas on the eastern shore either within or outside the Bay; high chlorophyll fluorescence is confined to the central and western Bay. Density data suggest a three-layered circulation pattern at the mouth of the Bay during the upwelling relaxation involving an inflow both at the surface and bottom and outflow at mid-depth. Associated with this exchange is an active high chlorophyll transport from the Bay to the adjacent coastal waters at mid-depth and inflow of low chlorophyll water from the adjacent shelf at the surface and near the bottom.

Dagoberto F. Arcos

Papers

The nearshore zone during coastal upwelling: Daily variability and coupling between primary and secondary production off central Chile

The jack mackerel fishery and El Niño 1997–98 effects off Chile

Seasonal growth of small pelagic fish off Talcahuano, Chile (37°S, 73°W): a consequence of their reproductive strategy to seasonal upwelling?

Recruitment of common sardine (Strangomera bentincki) and anchovy (Engraulis ringens) off central-south Chile in the 1990s and the impact of the 1997-1998 El Niño

Upwelling and the distribution of chlorophyll a within the Bay of Concepción, Chile