pCO2 effects on species composition and growth of an estuarine phytoplankton community

Geographical and seasonal patterns in the carbonate chemistry of Narragansett Bay, RI

This study examined geographical and seasonal patterns in carbonate chemistry and will facilitate assessment of acidification conditions and the current state of the seawater carbonate chemistry system in Narragansett Bay. Direct measurements of total alkalinity, dissolved inorganic carbon, dissolved oxygen percent saturation, water temperature, salinity and pressure were performed during monthly sampling cruises carried out over three years. These measurements were used to calculate the following biologically relevant carbonate system parameters: total pH ( p H T ) , the partial pressure of carbon dioxide in the gas phase p C O 2 , and the aragonite saturation state Ω A . The information provided by carbonate chemistry analysis allowed for the characterization of acidification events which have the potential to disrupt the species composition and ecological functioning of coastal biological communities and threaten commercially important aquatic life. We found very robust relationships between salinity and total alkalinity R adjusted 2 = 0.82 and between salinity and dissolved inorganic carbon R adjusted 2 = 0.81 that persisted through all regions, seasons, and depth-layers with mixing of coastal waters with freshwater entering in the upper bay being an important driver on alkalinity and dissolved inorganic carbon distributions. We compared the metabolically linked calculated carbonate system parameters with dissolved oxygen (DO) saturation and found high correlation, with DO percent saturation exhibiting robust correlation with the calculated carbonate system parameters total pH ( r = 0.70 ) and with partial pressure of carbon dioxide in the gas phase ( r = - 0.71 ) . Using a statistical model to correct for the confounded effects of time and space that are a common challenge in marine survey design, we found that acidification events occurred in the Northern Region of the bay, primarily during the Summer and Fall, and likely due to a combination of microbial respiration and stratification. These acidification events, especially in the Northern Region, have the potential to adversely impact aquatic life.

Responses of marine ecosystems to climate change impacts and their treatment in biogeochemical ecosystem models

To predict the effects of climate change on marine ecosystems and the effectiveness of intervention and mitigation strategies, we need reliable marine ecosystem response models such as biogeochemical models that reproduce climate change effects. We reviewed marine ecosystem parameters and processes that are modified by climate change and examined their representations in biogeochemical ecosystem models. The interactions among important aspects of marine ecosystem modelling are not often considered due to complexity: these include the use of multiple IPCC scenarios, ensemble modelling approach, independent calibration datasets, the consideration of changes in cloud cover, ocean currents, wind speed, sea-level rise, storm frequency, storm intensity, and the incorporation of species adaptation to changing environmental conditions. Including our recommendations in future marine modelling studies could help improve the accuracy and reliability of model predictions of climate change impacts on marine ecosystems.

Nitrogen isotope fractionation in a continuous culture system containing phytoplankton and blue mussels

An experiment was conducted to examine the fractionation of nitrogen stable isotopes in a continuous culture system containing field collected estuarine phytoplankton and blue mussels, Mytilus edulis. Nitrate and phosphate were added to culture vessels at concentrations above ambient levels and nitrogen isotope ratios (δ¹⁵N) were measured in particulate matter (PM) and blue mussels over the course of the 15-day experiment. The added nutrients resulted in large productivity and chlorophyll increases in the system. Study results indicate that rapid and significant nitrogen isotope fractionation can occur during incorporation by phytoplankton grown under conditions of excess dissolved inorganic nitrogen, as shown by δ¹⁵N values depleted by as much as 9‰ in PM from the higher nutrient treatments. These lower δ¹⁵N values were also reflected in mussels exposed to culture vessels effluents. Therefore, nitrogen concentration needs to be considered when using δ¹⁵N values in biota as indicators of anthropogenic nitrogen inputs.

The interactive effects of stratospheric ozone depletion, UV radiation, and climate change on aquatic ecosystems

Summary of current knowledge about effects of UV radiation in inland and oceanic waters related to stratospheric ozone depletion and climate change.

Dual role of DOM in a scenario of global change on photosynthesis and structure of coastal phytoplankton from the South Atlantic Ocean

We evaluated the dual role of DOM (i.e., as a source of inorganic nutrients and as an absorber of solar radiation) on a phytoplankton community of the western South Atlantic Ocean. Using a combination of microcosms and a cluster approach, we simulated the future conditions of some variables that are highly influenced by global change in the region. We increased nutrients (i.e., anthropogenic input) and dissolved organic matter (DOM), and we decreased the pH, to assess their combined impact on growth rates (μ), species composition/abundance and size structure, and photosynthesis (considering in this later also the effects of light quality i.e., with and without ultraviolet radiation). We simulated two Future conditions (Fut) where nutrients and pH were similarly manipulated, but in one the physical role of DOM (Fut_out) was assessed whereas in the other (Fut_in) the physico-chemical role was evaluated; these conditions were compared with a control (Present condition, Pres). The μ significantly increased in both Fut conditions as compared to the Pres, probably due to the nutrient addition and acidification in the former. The highest μ were observed in the Fut_out, due to the growth of nanoplanktonic flagellates and diatoms. Cells in the Fut_in were photosynthetically less efficient as compared to those of the Fut_out and Pres, but these physiological differences, also between samples with or without solar UVR observed at the beginning of the experiment, decreased with time hinting for an acclimation process. The knowledge of the relative importance of both roles of DOM is especially important for coastal areas that are expected to receive higher inputs and will be more acidified in the future.

Herbivorous protist growth and grazing rates at in situ and artificially elevated temperatures during an Arctic phytoplankton spring bloom

To assess protistan grazing impact and temperature sensitivity on plankton population dynamics, we measured bulk and species-specific phytoplankton growth and herbivorous protist grazing rates in Disko Bay, West Greenland in April-May 2011. Rate estimates were made at three different temperatures in situ (0 °C), +3 °C and +6 °C over ambient. In situ Chlorophyll a (Chl a) doubled during the observation period to ∼12  µg Chl a L-1, with 60-97% of Chl a in the >20 µm size-fraction dominated by the diatom genus Chaetoceros. Herbivorous dinoflagellates comprised 60-80% of microplankton grazer biomass. At in situ temperatures, phytoplankton growth or grazing by herbivorous predators <200 µm was not measurable until 11 days after observations commenced. Thereafter, phytoplankton growth was on average 0.25 d-1. Phytoplankton mortality due to herbivorous grazing was only measured on three occasions but the magnitude was substantial, up to 0.58 d-1. Grazing of this magnitude removed ∼100% of primary production. In short-term temperature-shift incubation experiments, phytoplankton growth rate increased significantly (20%) at elevated temperatures. In contrast, herbivorous protist grazing and species-specific growth rates decreased significantly (50%) at +6 °C. This differential response in phytoplankton and herbivores to temperature increases resulted in a decrease of primary production removed with increasing temperature. Phaeocystis spp. abundance was negatively correlated with bulk grazing rate. Growth and grazing rates were variable but showed no evidence of an inherent, low temperature limitation. Herbivorous protist growth rates in this study and in a literature review were comparable to rates from temperate waters. Thus, an inherent physiological inhibition of protistan growth or grazing rates in polar waters is not supported by the data. The large variability between lack of grazing and high rates of primary production removal observed here and confirmed in the literature for polar waters implies larger amplitude fluctuations in phytoplankton biomass than slower, steady grazing losses of primary production.

Carbon Stable Isotope Values in Plankton and Mussels Reflect Changes in Carbonate Chemistry Associated with Nutrient Enhanced Net Production

Coastal ecosystems are inherently complex and potentially adaptive as they respond to changes in nutrient loads and climate. We documented the role that carbon stable isotope (δ¹³C) measurements could play in understanding that adaptation with a series of three Ecostat (i.e., continuous culture) experiments. We quantified linkages among δ¹³C, nutrients, carbonate chemistry, primary, and secondary production in temperate estuarine waters. Experimental culture vessels (9.1 L) containing 33% whole and 67% filtered (0.2 μm) seawater were amended with dissolved inorganic nitrogen (N) and phosphorous (P) in low (3 vessels; 5 μM N, 0.3 μM P), moderate (3 vessels; 25 μM N, 1.6 μM P), and high amounts (3 vessels; 50 μM N, 3.1 μM P). The parameters necessary to calculate carbonate chemistry, chlorophyll-a concentrations, and particulate δ¹³C values were measured throughout the 14 day experiments. Outflow lines from the experimental vessels fed 250 ml containers seeded with juvenile blue mussels (Mytilus edulis). Mussel subsamples were harvested on days 0, 7, and 14 and their tissues were analyzed for δ¹³C values. We consistently observed that particulate δ¹³C values were positively correlated with chlorophyll-a, carbonate chemistry, and to changes in the ratio of bicarbonate to dissolved carbon dioxide ( [Formula: see text] :CO₂). While the relative proportion of [Formula: see text] to CO₂ increased over the 14 days, concentrations of each declined, reflecting the drawdown of carbon associated with enhanced production. Plankton δ¹³C values, like chlorophyll-a concentrations, increased over the course of each experiment, with the greatest increases in the moderate and high treatments. Trends in δ¹³C over time were also observed in the mussel tissues. Despite ecological variability and different plankton abundances the experiments consistently demonstrated how δ¹³C values in primary producers and consumers reflected nutrient availability, via its impact on carbonate chemistry. We applied a series of mixed-effects models to observational data from Narragansett Bay and the model that included in situ δ¹³C and percent organic matter was the best predictor of [ [Formula: see text]]. In temperate, plankton-dominated estuaries, δ¹³C values in plankton and filter feeders reflect net productivity and are a valuable tool to understand the production conditions under which the base of the food chain was formed.