Virginia Villafañe

Summary: Browning and nutrient inputs from extreme rainfall, together with increased vertical mixing due to strong winds, are more frequent in coastal ecosystems; however, their interactive effects on phytoplankton are poorly understood. We conducted experiments to quantify how browning, together with different mixing speeds (fluctuating radiation), and a nutrient pulse alter primary productivity and photosynthetic efficiency in estuarine phytoplankton communities. Phytoplankton communities (grazers excluded) were exposed simultaneously to these drivers, and key photosynthetic targets were quantified: oxygen production, electron transport rates (ETRs), and carbon fixation immediately following collection and after a 2‐d acclimation/adaptation period. Increasing mixing speeds in a turbid water column (e.g. browning) significantly decreased ETRs and carbon fixation in the short term. Acclimation/adaptation to this condition for 2 d resulted in an increase in nanoplanktonic diatoms and a community that was photosynthetically more efficient; however, this did not revert the decreasing trend in carbon fixation with increased mixing speed. The observed interactive effects (resulting from extreme rainfall and strong winds) may have profound implications in the trophodynamics of highly productive system such as the Southwest Atlantic Ocean due to changes in the size structure of the community and reduced productivity.

Coastal areas can be defined as the interface zones between land and sea, with terrestrial environments deeply influencing marine ones and vice versa (Gattuso et al. 1998, Martinez et al. 2007). Although these areas do not have strict spatial boundaries, their seaward limit is often set to include the continental shelf, commonly defined as to the isobath of 200 m (Spalding et al. 2007). Approximately 60% of the human population lives now within 100 km of the coasts (Lotze et al. 2006, Halpern et al. 2008). Why are these areas so attractive for human settlement even since pre-historic times? One of the most probable answers is because of the favorable biophysical and climatic conditions together with the ease of communication and navigation that these sites frequently offer. In fact, many of the world’s large cities are located in coastal areas, and a large portion of the economic activities are concentrated there (Costanza et al. 1997). Moreover, coastal areas harbor rich and valuable natural resources that provide important goods (e.g., food and raw materials) and services (e.g., disturbance regulation and nutrient cycling) for human beings. Due to their high primary productivity, these areas serve as feeding and spawning grounds and nurseries for many fish and invertebrates (including many of commercial interest) (Elliott and Dewailly 1995, Beck et al. 2001) therefore supporting high secondary production (ICES 2008).

Increases in ultraviolet radiation (UVR) levels due to the ongoing stratification of water bodies and higher nutrient concentrations either through riverine or aeolian‐dust‐inputs are expected in the near future in coastal surface waters. Here, we combined remote‐sensing data of particulate organic carbon (POC; 1997–2016 period), observational data of solar radiation (1999–2015 period), and a mid‐term experimental approach with coastal plankton communities from South Atlantic Ocean (SAO) to test how the interaction between increased nutrients by riverine and aeolian‐dust inputs and high UVR may alter the community dynamics and the CO₂ sink capacity of these ecosystems in the future. Our results show a decline ∼ 27% in the sink capacity of the coastal ecosystems regardless of the nutrient source considered and under high UVR levels. This decreased CO₂ uptake was coupled with a high dynamic photoinhibition and dark recovery of photosystem II and shifts in the community structure toward the dominance by nano‐flagellates. Moreover, remote‐sensing data also evidences an incipient tipping point with decreasing POC values in this area over the annual planktonic succession. Therefore, we propose that to continue this climate and human‐mediated pressure, these metabolic responses could be strengthened and extended to other productive coastal areas.

The effects of ultraviolet radiation (UVR) under future expected conditions of acidification and increase in nutrient inputs were studied on a post-bloom phytoplankton and bacterioplankton community of Patagonian coastal waters. We performed an experiment using microcosms where two environmental conditions were mimicked using a cluster approach: present (ambient nutrients and pH) and future (increased nutrients and acidification), and acclimating the samples for five days to two radiation treatments (full solar radiation [+UVR] and exclusion of UVR [–UVR]). We evaluated the short-term (hours) sensitivity of the community to solar UVR through chlorophyll a fluorescence parameters (e.g. the effective photochemical quantum yield of PSII [Φ_PSII]) at the beginning, at the mid-point and at the end of the acclimation period. Primary production and heterotrophic bacterial production (HBP) were determined, and biological weighting functions were calculated, at the beginning and at the end of the acclimation period. Mid-term effects (days) were evaluated as changes in taxonomic composition, growth rates and size structure of the community. Although the UVR-induced inhibition on Φ_PSII decreased in both clusters, samples remained sensitive to UVR after the 5 days of acclimation. Also, under the future conditions, there was, in general, an increase in the phytoplankton carbon incorporation rates along the experiment as compared to the present conditions. Bacterioplankton sensitivity to UVR changed along the experiment from inhibition to enhancement of HBP, and future environmental conditions stimulated bacterial growth, probably due to indirect effects caused by phytoplankton. Those changes in the microbial loop functioning and structure under future global change conditions might have important consequences for the carbon pump and thus for the carbon sequestration and trophodynamics of Patagonian coastal waters.