Description: At each study station, surface water temperature (±0.2°C) and salinity (±0.06) were measured in situ by triplicate readings with a multiparameter probe (Horiba U-10, Japan). Using Niskin bottles, surface water samples were collected for quantification of plankton and estimations of chlorophyll a. For the former, 250 ml were fixed with Lugol's solution (1% final concentration) and kept in the dark until analysis under the microscope, while for chlorophyll a, a volume of 200–250 mL was filtrated onboard through filter GF/C and kept at -20°C.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. Carbon content was calculated following the method of Menden-Deuer et al. (2000) in which biovolume was estimated assigning a geometrical shape to each species (Hillebrand et al., 1999). The biomass is the result of multiplying the carbon content of a species by its abundance in the sample. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.

Description: The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size 〉 5 µm) and in situ chorophyll a were assessed in a shallow (〈50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification) were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.

Description: Parasites of phytoplankton influence phytoplankton bloom dynamics and may severely affect the type of food available for higher trophic levels. The incidence of parasitic infections generally is expected to increase across ecosystems worldwide under the scenario of global change. Herein we report on a massive parasite infection on two dominant diatoms of the austral winter bloom, namely Thalassiosira pacifica and Chaetoceros diadema, recorded during an extreme precipitation period in the Bahía Blanca Estuary, Argentina. The parasite infection was concomitant with a marked drop in water salinity and affected more than 40% of host cells. Although the parasite on C. diadema was not identified, the parasite on T. pacifica was most likely Pirsonia sp., a nanoflagellate with high host specificity. After the intense rainy period and the parasitic infection, the phytoplankton biomass dropped (by more than 80%) and the community structure shifted to one with smaller species (i.e. Thalassiosira curviseriata, T. hibernalis and T. minima). We discuss the implications that these modifications may have on the food web dynamics and the potential relationship between precipitation-driven modifications in water properties and the emergence of parasitism in coastal eutrophic environments.

Description: Highlights • Increasing influence of multiple environmental drivers produces changes on the temporal variability of species. • The intensity and hierarchy of drivers acting upon organisms within alternative regimes of variability may differ. • We identified regimes of variability of phytoplankton and depicted cascading effects of multiple drivers in each period. • The number of factors driving the response of phytoplankton increased along time and produced the erosion of productivity patterns. • The hierarchy and interactions of drivers changed over time, revealing that management policies require constant update. Abstract Estuaries are among the most valuable aquatic systems by their services to human welfare. However, increasing human activities at the watershed along with the pressure of climate change are fostering the co-occurrence of multiple environmental drivers, and warn of potential negative impacts on estuaries resources. At present, no clear understanding of how coastal ecosystems will respond to the non-stationary effect of multiple drivers. Here we analysed the temporal interaction among multiple environmental drivers and their changing priority on shaping phytoplankton response in the Bahía Blanca Estuary, SW Atlantic Ocean. The interaction among environmental drivers and the number of significant direct and indirect effects on chlorophyll concentration increased over time in concurrence with enhanced anthropogenic stress, changing winter climate and wind patterns. Over the period 1978–1993, proximal variables such as nutrients, water temperature and salinity, showed a dominant effect on chlorophyll, whereas in more recent years (1993–2009) climate signals (SAM and ENSO) boosted indirect effects through its influence on precipitation, wind, water temperature and turbidity. Turbidity emerged as the dominant driver of chlorophyll while in recent years acted synergistically with the concentration of dissolved nitrogen. As a result, chlorophyll concentration showed a significant negative trend and a loss of seasonal peaks reflecting a pronounced reorganisation of the phytoplankton community. We stress the need to account for the changing priority of drivers to understand, and eventually forecast, biological responses under projected scenarios of global anthropogenic change.

Description: We examined austral summer phytoplankton data (December–March) that cover the years 1978–2008 and compared with physico-chemical and meteorological variations in the Bahía Blanca Estuary, Argentina. During the years 1978–1982, 1992–1993 and 2006–2008, counts of phytoplankton abundance showed an increase in recent summers; from a mean value of 12×103 cells L−1 in 1978 to 2239×103 cells L−1 in 2008, while the chlorophyll concentration remained relatively constant (8.5±2.5 μg L−1, CV=27%) over the continuous time series (1978–2008). The rise in the ratio ‘cell abundance: chlorophyll concentration’ was linked to modifications in species composition, from dominance of phytoflagellates (10–20 μm) and relatively large diatoms (e.g., Cyclotella striata 25–38 μm, Paralia sulcata 15–70 μm, Cerataulina pelagica 18–30 μm, Thalassiosra hendeyi 27–52 μm) towards the dominance of the small (5–15 μm) centric diatom Thalassiosira minima, which reached 〉80% of the total phytoplankton abundance in summers 2006–2008. The Bahía Blanca Estuary has undergone climate modifications and increasing anthropogenic disturbances during the last three decades. In the early 1990s, regional climatic conditions revealed a significant shift. Additionally, dredging activities were initiated to allow the traffic of large ships, rising the levels of suspended sediments, and the invasive copepod Eurytemora americana was introduced via ballast waters into the estuary and has displaced the dominance of the native copepod Acartia tonsa towards summer periods. The examination of physico-chemical conditions of the estuary showed a trend to increase in the minima of water temperature and higher water turbidity, dissolved phosphate, nitrite and nitrate concentrations in the pelagic environment in recent summers. We discuss the potential effects of these changes and trophic interactions on the structure and composition of the phytoplankton summer blooms in this temperate and eutrophic estuary in the Southwestern Atlantic. Highlights: ► We analyzed phytoplankton summer bloom dynamics in a temperate estuary during 1978–2008. ► Community structure and species composition has changed in recent summers. ► Hydroclimatic shift and anthropogenic disturbances have affected the physico-chemical conditions. ► We registered changes in water turbidity, temperature, nutrient concentrations and trophodynamics. ► We discuss the potential implications of phytoplankton restructuration in the pelagic food web.

Description: Quantifying biotic feedbacks in response to environmental signals is fundamental to assess ecosystem perturbation. We analyzed the joint effects of eutrophication, derived from sewage pollution, and climate at the base of the pelagic food web in the Bahía Blanca Estuary (SW Atlantic Ocean). A two-year survey of environmental conditions and microplankton communities was conducted in two sites affected by contrasting anthropogenic eutrophication conditions. Under severe eutrophication, we found higher phytoplankton abundance consistently dominated by smaller sized, non siliceous species, while microzooplankton abundance remained lower and nutrient stoichiometry showed conspicuous deviations from the Redfield ratio. Phytoplankton growth in such conditions appeared controlled by phosphorous. In turn, microplankton biomass and phytoplankton size ratio (〈20. μm:〉20. μm) displayed a saturation relationship with nutrients in the highly eutrophic area, although mean phytoplankton growth was similar in both eutrophic systems. The strength of links within the estuarine network, quantified through path analysis, showed enhanced relationships under larger anthropogenic eutrophication, which fostered the climate influence on microplankton communities. Our results show conspicuous effects of severe sewage pollution on the ecological stoichiometry, i.e., N and P excess with respect to Si, altering nutrient ratios for microplankton communities. This warns on wide consequences on food web dynamics and ultimately in ecosystem assets of coastal pelagic environments.

Description: Understanding phytoplankton species-specific responses to multiple biotic and abiotic stressors is fundamental to assess phenological and structural shifts at the community level. Here, we present the case of Thalassiosira curviseriata, a winter-blooming diatom in the Bahía Blanca Estuary, Argentina, which displayed a noticeable decrease in the past decade along with conspicuous changes in phenology. We compiled interannual field data to assess compound effects of environmental variations and grazing by the invasive copepod Eurytemora americana. The two species displayed opposite trends over the period examined. The diatom decreased toward the last years, mainly during the winters, and remained relatively constant over the other seasons, while the copepod increased toward the last years, with an occurrence restricted to winter and early spring. A quantitative assessment by structural equation modeling unveiled that the observed long-term trend of T. curviseriata resulted from the synergistic effects of environmental changes driven by water temperature, salinity, and grazing. These results suggest that the shift in the abundance distribution of T. curviseriata toward higher annual ranges of temperature and salinity—as displayed by habitat association curves—constitutes a functional response to avoid seasonal overlapping with its predator in late winters. The observed changes in the timing and abundance of the blooming species resulted in conspicuous shifts in primary production pulses. Our results provide insights on mechanistic processes shaping the phenology and structure of phytoplankton blooms.

Description: Marine microbial plankton hold high structural and functional diversity, however, high-resolution data are lacking in a large part of the Global Ocean, such as in subpolar areas of the SW Atlantic. The Burdwood Bank (BB) is a submerged plateau (average depth 100 m) that constitutes the westernmost segment of the North Scotia Ridge (54°–55°S; 56°–62°W). The BB hosts rich benthic biodiversity in low chlorophyll waters of the southern Patagonian Shelf, Argentina, declared Namuncurá Marine Protected Area (NMPA) in 2013. So far, the pelagic microorganisms above the bank have not been described. During austral summer 2016, we assessed the microbial plankton (0.2–200 μm cell size) biomass and their taxonomical and functional diversity along a longitudinal transect (54.2–55.3°S, 58–68°W) from the Beagle Channel (BC) to the BB, characterized by contrasting hydrography. Results displayed a marked zonation in the composition and structure of the microbial communities. The biomass of phytoplankton 〉5 μm was 28 times higher in the BC, attributed mainly to large diatom blooms, than in oceanic waters above the BB, where the small coccolithophore Emiliania huxleyi and flagellates 〈10 μm dominated. In turn, the biomass of microheterotrophs above the BB doubled the biomass in the BC due to large ciliates. Notably, toxic phytoplankton species and their phycotoxins were detected, in particular high abundance of Dinophysis acuminata and pectenotoxins above the bank, highlighting their presence in open subpolar regions. Picophytoplankton (〈2 μm), including Synechococcus and picoeukaryotes, were remarkably important above the BB, both at surface and deep waters (up to 150 m). Their biomass surpassed by 5 times that of phytoplankton 〉 5 μm, emphasizing the importance of small-sized phytoplankton in low chlorophyll waters. The homogeneous water column and high retention above the bank seem to favor the development of abundant picophytoplankton and microzooplankton communities. Overall, our findings unfold the plankton configuration in the Southern Patagonian Shelf, ascribed as a sink for anthropogenic CO2, and highlight the diverse ecological traits that microorganisms develop to adjust their yield to changing conditions.