Planktonic habitats in the Amazon Plume region of the Western Tropical North Atlantic

The Western Tropical North Atlantic is a highly dynamic marine system where the Amazon River Plume (ARP) generates a patchwork of environmental conditions that favor different phytoplankton groups. To study phytoplanktonic community structure in such heterogeneous conditions, we used a set of five standard ship-based measurements taken from oceanographic surveys between 2010 and 2021 to characterize different habitat types. We then utilized a variety of multiparametric approaches to examine phytoplankton biodiversity in the different habitats to assess the biological relevance of our delineated habitats. Our approach generated a consistent set of habitat types across cruises carried out in multiple different years and the Amazon’s two predominant (wet and dry) seasons. Our phytoplankton community analyses revealed strong distinctions among all habitats along the plume gradient using in-vivo fluorescence and diagnostic pigments, and clear contrasts of diazotroph community along the mesohaline waters using direct cell-count, a pattern consistent with niche partitioning among similar species. The few apparent mismatches we found between phytoplankton community composition and habitat may reflect recent hydrographic changes driven by mixing and/or upwelling and thus may be a useful index to biologically-relevant temporal variation. Our habitat classification approach is straightforward and broadly applicable in identifying biologically distinct areas within heterogeneous and dynamic regions of the ocean. Keywords: Amazon River plume, Western Tropical North Atlantic, phytoplankton, habitat classification, hierachical clusterin

Sediment solid phase characteristics on the Amazon shelf obtained in April/May 2021 during RV METEOR cruise M174

The Amazon River is known as a region of intense biochemical reactivity. As a result of increased anthropogenic impacts in its catchment (e.g. population growth, agriculture, and deforestation), The Amazon River is undergoing fundamental changes. The RV METEOR cruise M174 (https://doi.org/10.1594/PANGAEA.935041) aimed to provide an integrated overview of nitrogen cycling processes in areas influenced by the Amazon River, which is crucial to estimate the ecosystem's health and productivity. During this cruise, a MUC was deployed at eight stations to collect short sediment cores, in order to explore the influence of the river on sediments. This dataset presents the results of sediment solid phase characteristics from the surface sediment down to 30 cm depth

Nutrient fluxes at the sediment-water interface in the Amazon shelf obtained in April/May 2021 during RV METEOR cruise M174

The Amazon River is known as a region of intense biochemical reactivity. As a result of increased anthropogenic impacts in its catchment (e.g. population growth, agriculture and deforestation), The Amazon River is undergoing fundamental changes. The RV METEOR cruise M174 (https://doi.org/10.1594/PANGAEA.935041) aimed to provide an integrated overview of nitrogen cycling processes in areas influenced by the Amazon River, which is crucial to estimate the ecosystem's health and productivity. During this cruise, a MUC was deployed at eight stations to collect short sediment cores, in order to explore the influence of the river on sediments. This dataset presents the results of nutrient fluxes at the sediment-water interface measured via incubation experiments

Porewater nutrients in sediment cores from the Amazon shelf obtained in April/May 2021 during RV METEOR cruise M174

The Amazon River is known as a region of intense biochemical reactivity. As a result of increased anthropogenic impacts in its catchment (e.g. population growth, agriculture, and deforestation), The Amazon River is undergoing fundamental changes. The RV METEOR cruise M174 (https://doi.org/10.1594/PANGAEA.935041) aimed to provide an integrated overview of nitrogen cycling processes in areas influenced by the Amazon River, which is crucial to estimate the ecosystem's health and productivity. During this cruise, a MUC was deployed at eight stations to collect short sediment cores, in order to explore the influence of the river on sediments. This dataset presents the results of porewater nutrients collected with rhizons and measured on board with an autoanalyzer (Quaatro seal analytics)

Seabird-Derived Nutrients Supply Modulates the Trophic Strategies of Mixotrophic Corals

International audienceThe ability of corals to modulate their nutrition strategy in response to variable nutrient supply remains poorly understood, limiting our understanding of energy flow in coral reef ecosystems and thus our comprehension of their resilience to global changes. We used a naturally occurring nutrient gradient along the reef flat of two seabird-inhabited islets in the SW Pacific to characterize spatiotemporal fluctuations in coastal nutrient availability, and how it modulates the trophic response of the mixotrophic coral Pocillopora damicornis . The clear gradients in dissolved [NOx] and δ 15 N values of macroalgae and both P. damicornis tissues and symbionts observed along the reef flat during the dry and the rainy season revealed that seabird-derived-N is supplied year-round to the reef flat. Yet, nitrogen isotope values of macroalgae show that the seabirds’ effect on coral reefs varies with sites and seasons. Metrics derived from the SIBER framework revealed that coral nutrition seasonally favored autotrophy when exposed to higher seabird guano concentrations and at inshore stations, while heterotrophy dominated in corals less exposed to seabird-derived nutrient supply. P. Damicornis is therefore able to cope with large changes in nitrogen supply induced by seabird island communities by switching between autotrophy and heterotrophy. These results shed light on the flexibility of resource sharing within the coral-algae symbiosis and highlight the importance of seabird populations to the functioning of coral reef ecosystems

Seabird-Derived Nutrients Supply Modulates the Trophic Strategies of Mixotrophic Corals

The ability of corals to modulate their nutrition strategy in response to variable nutrient supply remains poorly understood, limiting our understanding of energy flow in coral reef ecosystems and thus our comprehension of their resilience to global changes. We used a naturally occurring nutrient gradient along the reef flat of two seabird-inhabited islets in the SW Pacific to characterize spatiotemporal fluctuations in coastal nutrient availability, and how it modulates the trophic response of the mixotrophic coral Pocillopora damicornis. The clear gradients in dissolved [NOx] and δ15N values of macroalgae and both P. damicornis tissues and symbionts observed along the reef flat during the dry and the rainy season revealed that seabird-derived-N is supplied year-round to the reef flat. Yet, nitrogen isotope values of macroalgae show that the seabirds’ effect on coral reefs varies with sites and seasons. Metrics derived from the SIBER framework revealed that coral nutrition seasonally favored autotrophy when exposed to higher seabird guano concentrations and at inshore stations, while heterotrophy dominated in corals less exposed to seabird-derived nutrient supply. P. Damicornis is therefore able to cope with large changes in nitrogen supply induced by seabird island communities by switching between autotrophy and heterotrophy. These results shed light on the flexibility of resource sharing within the coral-algae symbiosis and highlight the importance of seabird populations to the functioning of coral reef ecosystems

Tracing the fate of seabird‐derived nitrogen in a coral reef using nitrate and coral skeleton nitrogen isotopes

International audienceSeabirds transfer nutrients from the ocean to their nesting island, potentially altering nitrogen (N) cycling within adjacent terrestrial and marine ecosystems. Yet, the processes involved in seabird‐N transfer along the land–sea continuum remain elusive. Using δ 15 N and δ 18 O measurements of groundwater nitrate, we demonstrate the role of brackish groundwater located within a coral island's landmass as a major reservoir of nitrate (at millimolar levels). Nearly all of the total dissolved seabird‐derived N leaching into the groundwater (mostly ammonium and uric acid) is converted to nitrate by nitrification, as supported by the relatively low δ 18 O of the groundwater nitrate (3.97‰ ± 0.30‰). Comparison of nitrate δ 15 N and δ 18 O suggests that little denitrification takes place within the groundwater lens, implying that the high δ 15 N of groundwater nitrate (13.73‰ ± 0.05‰) derives from the high trophic position of seabirds and postdepositional processes that increase the δ 15 N of seabird excreta. Seawater and coral skeleton samples from a reef flat exposed to groundwater had higher δ 15 N values than at sites devoid of groundwater influence, indicating that the main source of N at the latter site was the Subtropical Upper Water, while the groundwater nitrate dominated the exposed reef flat N pool up to 200 m from shore. In addition, these results indicate that coral‐bound δ 15 N can detect seabird‐derived nitrate δ 15 N, raising opportunities to reconstruct historical seabird‐N inputs to coral reefs in relation to climatic and other changes, such as the introduction of invasive species