Assessing the impact of nutrient loads on eutrophication in the semi-enclosed Izmir Bay combining observations and coupled hydrodynamic-ecosystem modelling

Intense human activities may strongly affect coastal environments threatening natural, societal and economic resources. In order to propose adequate measures to preserve coastal marine areas, a thorough understanding of their physical and biogeochemical features is required. This study focuses on one such coastal area, Izmir Bay located in the Eastern Mediterranean Sea. Izmir Bay is a highly populated area subject to many human induced stressors such as pollution and eutrophication, that has been suffering high nutrient loads for decades. Despite the construction of the Çiğli waste water treatment plant in 2000-2001 to reduce eutrophication, such pressures continue to occur. To study the current physical and biogeochemical dynamics of Izmir Bay and their spatial and temporal variability, a three-dimensional coupled hydrodynamic-ecosystem model (Delft3D modelling suite’s FLOW and ECO modules) is implemented. Using the model, the effect of excessive inorganic nutrient loading on the marine ecosystem as the main cause of this eutrophication is explored in an effort to advise on mitigation efforts for the Bay focusing on eliminating eutrophication. Results of different model scenarios show that the Inner and Middle Bay are nitrogen-limited while the Outer Bay is phosphorus-limited. Inner regions are more sensitive to variations in inorganic nitrogen input due to the low (<16) N/P ratio of nutrients in seawater. An increase in inorganic nitrogen triggers eutrophication events with primary production as an immediate response. Conversely, the Outer Bay ecosystem with N/P ratios above 16 is more sensitive to phosphate inputs, of which an increase causes a considerable enhancement in algal production. This study shows the vulnerability of Izmir Bay to anthropogenic nutrient input and model simulations indicate that management plans should consider reducing DIN discharges both in the inner-middle zones of Izmir Bay as well as inputs from the Gediz River. Additionally, phosphate inputs should be reduced to avoid an overall increase of algal production in the Outer Bay, the larger part of Izmir Bay.

Assessing Spatial Variation in Algal Productivity in a Tropical River Floodplain Using Satellite Remote Sensing

Studies of tropical floodplains have shown that algae are the primary source material for higher consumers in freshwater aquatic habitats. Thus, methods that can predict the spatial variation of algal productivity provide an important input to better inform management and conservation of floodplains. In this study, a prediction of the spatial variability in algal productivity was made for the Mitchell River floodplain in northern Australia. The spatial variation of aquatic habitat types and turbidity were estimated using satellite remote sensing and then combined with statistical modelling to map the spatial variation in algal primary productivity. Open water and submerged plants habitats, covering 79% of the freshwater flooded floodplain extent, had higher rates of algal production compared to the 21% cover of emergent and floating aquatic plant habitats. Across the floodplain, the predicted average algal productivity was 150.9 ± 95.47 SD mg C m−2 d−1 and the total daily algal production was estimated to be 85.02 ± 0.07 SD ton C. This study provides a spatially explicit representation of habitat types, turbidity, and algal productivity on a tropical floodplain and presents an approach to map ‘hotspots’ of algal production and provide key insights into the functioning of complex floodplain–river ecosystems. As this approach uses satellite remotely sensed data, it can be applied in different floodplains worldwide to identify areas of high ecological value that may be sensitive to development and be used by decision makers and river managers to protect these important ecological assets.

The importance of turbulent ocean-sea ice nutrient exchanges for simulation of ice algal biomass and production with CICE6.1 and Icepack 1.2

Different sea-ice models apply unique approaches in the computation of nutrient diffusion between the ocean and the ice bottom, which are generally decoupled from the calculation of turbulent momentum and heat flux. Often, a simple molecular diffusion formulation is used. We argue that nutrient transfer from the ocean to sea ice should be as consistent as possible with momentum and heat transfer, since all these fluxes respond to varying forcing in a similar fashion. We hypothesize that biogeochemical models which do not consider such turbulent nutrient exchanges between the ocean and the sea-ice underestimate bottom-ice algal production. The Los Alamos Sea Ice Model (CICE + Icepack) was used to test this hypothesis by comparing simulations with molecular and turbulent diffusion of nutrients into the bottom of sea ice, implemented in a way that is consistent with turbulent momentum and heat exchanges. Simulation results support the hypothesis, showing a significant enhancement of ice algal production and biomass when nutrient limitation was relieved by bottom-ice turbulent exchange. Our results emphasize the potentially critical role of turbulent exchanges to sea ice algal blooms, and the importance of thus properly representing them in biogeochemical models. The relevance of this becomes even more apparent considering ongoing trends in the Arctic Ocean, with a predictable shift from light to nutrient limited growth of ice algae earlier in the spring, as the sea ice becomes more fractured and thinner with a larger fraction of young ice with thin snow cover.

Consequences of Increased Variation in Peatland Hydrology for Carbon Storage: Legacy Effects of Drought and Flood in a Boreal Fen Ecosystem

Globally important carbon (C) stores in boreal peatlands are vulnerable to altered hydrology through changes in precipitation and runoff patterns, groundwater inputs, and a changing cryosphere. These changes can affect the extent of boreal wetlands and their ability to sequester and transform C and other nutrients. Variation in precipitation patterns has also been increasing, with greater occurrences of both flooding and drought periods. Recent work has pointed to the increasing role of algal production in regulating C cycling during flooded periods in fen peatlands, but exactly how this affects the C sink-strength of these ecosystems is poorly understood. We evaluated temporal trends in algal biomass, ecosystem C uptake and respiration (using static and floating chamber techniques), and spectroscopic indicators of DOM quality (absorbance and fluorescence) in a boreal rich-fen peatland in which water table position had been experimentally manipulated for 13 years. Superimposed on the water table treatments were natural variations in hydrology, including periods of flooding, which allowed us to examine the legacy effects of flooding on C cycling dynamics. We had a particular focus on understanding the role of algae in regulating C cycling, as the relative contribution of algal production was observed to significantly increase with flooding. Ecosystem measures of gross primary production (GPP) increased with algal biomass, with higher algal biomass and GPP measured in the lowered water table treatment two years after persistent flooding. Prior to flooding the lowered treatment was the weakest C sink (as CO2), but this treatment became the strongest sink after flooding. The lower degree of humification (lower humification index, HIX) and yet lower bioavailability (higher spectral slope ratio, Sr) of DOM observed in the raised treatment prior to flooding persisted after two years of flooding. An index of free or bound proteins (tyrosine index, TI) increased with algal biomass across all plots during flooding, and was lowest in the raised treatment. As such, antecedent drainage conditions determined the sink-strength of this rich fen—which was also reflected in DOM characteristics. These findings indicate that monitoring flooding history and its effects on algal production could become important to estimates of C balance in northern wetlands.

Factors Leading to Increased Algal Production in Mountain Lakes: A Paleolimnological Perspective from the Uinta Mountains, Utah, USA

&lt;p&gt;Mountain lakes are often remote, located in environments that experience cold temperatures, high incident solar and ultraviolet radiation, and prolonged ice and snow cover. They are, therefore, frequently dilute and oligotrophic. Together these factors can a&amp;#64256;ect mountain lake ecosystem structure, diversity, and productivity. However, distant human activities resulting in atmospheric pollution, as well as more local disturbances, such as fish stocking, potentially increase nutrient inputs and alter mountain lake ecosystems. Our research addresses how these human activities have altered algal production in Uinta Mountain (Utah, USA) lakes. Sedimentary chlorophyll a and its derivatives were measured using visible reflectance spectroscopy in short sediment cores from a total of 12 lakes, including both alpine and subalpine lakes, to determine trends in algal production. All sediment cores were dated using &lt;sup&gt;210&lt;/sup&gt;Pb and &lt;sup&gt;14&lt;/sup&gt;C dating, and the records were shown to extend back 300 to 500 years. Our results show that regardless of whether lakes were stocked or not, algal production remained virtually unchanged until 1950 when it increased dramatically in most lakes. The widespread distribution of the sites points to a regional stressor, such as atmospheric deposition of nutrients, as being the main cause for increased algal production. Additional analyses, including diatoms and C and N isotopes, measured in sediments from some lakes support this finding. The few lakes where algal production trends differed showed either that algal production had changed little overtime or that it was variable throughout the record. Although speculative, the lake that showed unchanged algal production is surrounded by a wetland that may have contributed nitrogen to the lake throughout the record meaning that additional nitrogen had little effect on algal production. Lakes with more variable algal production were subalpine lakes. The variable trend may point to more complex pathways and transport of nitrogen from the catchment to the lakes at lower elevation sites. Our findings show that remote mountain lakes, which typically are important water resources and biodiversity hotspots, are rapidly changing as a result of human activities, but not all of these lakes are responding in the same way. To effectively protect mountain lakes it will be important to identify and quantify influential factors affecting lake response to anthropogenic stressors.&amp;#160;&amp;#160;&lt;/p&gt;

Parsing the DOM sources using calibrated biomarkers in the San Francisco Bay Estuary

&lt;p&gt;The San Francisco Bay Estuary (SFBE) together with the Sacramento&amp;#8211;San Joaquin River Delta is the second largest estuary in the United States and represents a highly dynamic ecosystem. From 2014 to 2016, we conducted three transects across a salinity gradient to investigate the roles of sources, hydrologic and seasonal changes on the DOM composition. Sampling started with a riverine endmember, through a vast area of marshes, wetlands, to the Golden Gate, the largest estuary in western North America. The winter transect at its maximum discharge allowed the study of DOM dynamics largely in the absence of photodegradation processes and low levels of algal production; the summer transect captured significant photodegradation and algal production; the spring transect revealed the signal of stored DOM from the snowmelt cold water flows. Multiple studies indicated that algal primary production alone cannot support the SFBE foodweb, and the wetlands could also serve to reduce DOM loadings coming off of the delta.&amp;#160; Hence, other sources of organic matter must be considered, including autochthonous and allochthonous DOM. Terrestrial DOM export in SFBE were revealed by dissolved lignin dynamics. Optical proxies (UV-vis and fluorescence) were also used to study the photochemical and biological transformations of DOM.&lt;/p&gt;

Stress factors resulting from the Arctic vernal sea-ice melt

During sea-ice melt in the Arctic, primary production by sympagic (sea-ice) algae can be exported efficiently to the seabed if sinking rates are rapid and activities of associated heterotrophic bacteria are limited. Salinity stress due to melting ice has been suggested to account for such low bacterial activity. We further tested this hypothesis by analyzing samples of sea ice and sinking particles collected from May 18 to June 29, 2016, in western Baffin Bay as part of the Green Edge project. We applied a method not previously used in polar regions—quantitative PCR coupled to the propidium monoazide DNA-binding method—to evaluate the viability of bacteria associated with sympagic and sinking algae. We also measured cis-trans isomerase activity, known to indicate rapid bacterial response to salinity stress in culture studies, as well as free fatty acids known to be produced by algae as bactericidal compounds. The viability of sympagic-associated bacteria was strong in May (only approximately 10% mortality of total bacteria) and weaker in June (average mortality of 43%; maximum of 75%), with instances of elevated mortality in sinking particle samples across the time series (up to 72%). Short-term stress reflected by cis-trans isomerase activity was observed only in samples of sinking particles collected early in the time series. Following snow melt, however, and saturating levels of photosynthetically active radiation in June, we observed enhanced ice-algal production of bactericidal compounds (free palmitoleic acid; up to 4.8 mg L–1). We thus suggest that protection of sinking sympagic material from bacterial degradation early in a melt season results from low bacterial activity due to salinity stress, while later in the season, algal production of bactericidal compounds induces bacterial mortality. A succession of bacterial stressors during Arctic ice melt helps to explain the efficient export of sea-ice algal material to the seabed.