Toward a Solution of the “Peruvian Puzzle”: Pelagic Food-Web Structure and Trophic Interactions in the Northern Humboldt Current Upwelling System Off Peru

The northern Humboldt Current upwelling system (HCS) belongs to the most productive marine ecosystems, providing five to eight times higher fisheries landings per unit area than other coastal upwelling systems. To solve this “Peruvian puzzle”, to elucidate the pelagic food-web structure and to better understand trophic interactions in the HCS, a combined stable isotope and fatty acid trophic biomarker approach was adopted for key zooplankton taxa and higher trophic positions with an extensive spatial coverage from 8.5 to 16°S and a vertical range down to 1,000 m depth. A pronounced regional shift by up to ∼5‰ in the δ15N baseline of the food web occurred from North to South. Besides regional shifts, δ15N ratios of particulate organic matter (POM) also tended to increase with depth, with differences of up to 3.8‰ between surface waters and the oxygen minimum zone. In consequence, suspension-feeding zooplankton permanently residing at depth had up to ∼6‰ higher δ15N signals than surface-living species or diel vertical migrants. The comprehensive data set covered over 20 zooplankton taxa and indicated that three crustacean species usually are key in the zooplankton community, i.e., the copepods Calanus chilensis at the surface and Eucalanus inermis in the pronounced OMZ and the krill Euphausia mucronata, resulting in an overall low number of major trophic pathways toward anchovies. In addition, the semi-pelagic squat lobster Pleuroncodes monodon appears to play a key role in the benthic-pelagic coupling, as indicated by highest δ13C’ ratios of −14.7‰. If feeding on benthic resources and by diel vertical migration, they provide a unique pathway for returning carbon and energy from the seafloor to the epipelagic layer, increasing the food supply for pelagic fish. Overall, these mechanisms result in a very efficient food chain, channeling energy toward higher trophic positions and partially explaining the “Peruvian puzzle” of enormous fish production in the HCS.

Movement shapes the structure of fish communities along a cross-shore section in the California Current

Pelagic fish communities are shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes remains poorly understood. Here, we use a regional implementation of the APex ECOSystem Model (APECOSM), a mechanistic model of the pelagic food web, to investigate these processes in the California Current, a highly productive upwelling system characterized by vigorous mesoscale circulation. The model is coupled with an eddy-resolving representation of ocean currents and lower trophic levels, and is tuned to reproduce observed fish biomass from fisheries independent trawls. Several emergent properties of the model compare realistically with observations. First, the epipelagic community accounts for one order of magnitude less biomass than the vertically migratory community, and is composed by smaller species. Second, the abundance of small fish decreases from the coast to the open ocean, while the abundance of large fish remains relatively uniform. This in turn leads to flattening of biomass size-spectra away from the coast for both communities. Third, the model reproduces a cross-shore succession of small to large sizes moving offshore, consistent with observations of species occurrence. These cross-shore variations emerge in the model from a combination of: (1) passive offshore advection by the mean current, (2) active swimming towards coastal productive regions to counterbalance this transport, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. Our results highlight the importance of passive and active movement in structuring the pelagic food web, and suggest that a correct representation of these processes is needed for realistic simulations with marine ecosystem models.

Cladocera paleocommunity to disentangle the impact of anthropogenic and climatic stressors on a deep subalpine lake ecosystem (Lake Iseo, Italy)

In big lakes with strong anthropogenic pressure, it is usually difficult to disentangle the impacts of climate variability from those driven by eutrophication. The present work aimed at the reconstruction of change in the species distribution and density of subfossil Cladocera in Lake Iseo (Italy) in relation to climate and anthropogenic pressure. We related subfossil Cladocera species composition and density in an 80-cm sediment core collected in the pelagic zone of Lake Iseo to long-term temperature trends and phosphorus concentration inferred by diatoms frustules. The Cladocera remains detected in Lake Iseo sediment reflected the species composition and density of modern pelagic Cladocera assemblages. Cladocera rapidly respond to environmental change, and that climate change combined with eutrophication can induce changes in community composition and species density. At the beginning of twentieth century, when global warming was not yet so accentuated, the nutrient increase in water resulted as the principal driver in determining the long-term development of plankton communities and pelagic food web structure. Moreover, catchment-related processes may decisively affect both species composition and density of the lake planktonic communities due to the decrease of lake water transparency induced by input of inorganic material from the catchment area to the lake. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

Diel Vertical migration of marine organisms and the biological carbon pump

<p>Diel Vertical Migration (DVM) is a key feature of pelagic and mesopelagic ecosystems, mainly driven by predator-prey interactions along a time-varying vertical gradient of light. Marine organisms including meso-zooplankton and fish typically hide from visual predators at depth during daytime and migrate up at dusk to feed in productive near-surface waters during nighttime. Specific migration patterns, however, vary tremendously, for instance in terms of residency depth during day and night. In addition to environmental parameters such as light intensity and oxygen concentration, the migration pattern of each organism is intrinsically linked to the patterns of its conspecifics, its prey, and its predators through feedbacks that are hard to understand—but important to consider.</p><p>DVM not only affects trophic interactions, but also the biogeochemistry of the world’s oceans.  Organisms preying at the surface and actively migrating vertically transport carbon to depth, contributing to the biological carbon pump, and directly connecting surface production with mesopelagic and demersal ecosystems.</p><p>Here, we present a method based on a game-theoretic trait-based mechanistic model that enables the optimal DVM patterns for all organisms in a food-web to be computed simultaneously. The results are used to investigate the contributions of the different food-web pathways to the active component of the biological carbon pump. We apply the method to a modern pelagic food-web (comprised of meso- and macro-zooplankton, forage fish, mesopelagic fish, large pelagic fish and gelatinous organisms), shedding light on the direct effects that different trophic levels can have on the DVM behaviours of each other. The model is run on a global scale to assess the carbon export mediated by different functional groups, through fecal pellet production, carcasses sinking and respiration.</p><p>Finally, the model output is coupled to an ocean inverse circulation model to assess the carbon sequestration potential of the different export pathways. Results indicate that the carbon sequestration mediated by fish is much more important than presently recognised in global assessments of the biological carbon pump. The work we present relates to contemporary ecosystems, but we also explain how it can be adapted to fit any pelagic food-web structure to assess the contribution of the active biological pump to the global carbon cycle in past ecosystems.</p>