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The role of dissolved organic nitrogen (DON) on the development and extend of eutrophication-driven hypoxia and responses to global warming

Final Report Summary - DON-HYPO (The role of dissolved organic nitrogen (DON) on the development and extend of eutrophication-driven hypoxia and responses to global warming.)

Executive Summary

Coastal hypoxia is a mounting problem worldwide that has been recognised as a major threat to marine coastal biota. The decline in dissolved oxygen can affect ecosystems structure and functioning. Managerial efforts to prevent hypoxia and recover ecosystems that have already been affected by hypoxia are largely based on nutrient reduction plans. Most nutrient reduction plans are focused on the reduction of the inorganic fraction of the nutrient loadings, particularly in reductions of nitrate and phosphate inputs. However, the concentration of dissolved organic nitrogen (DON) frequently exceeds that of dissolved inorganic nitrogen (DIN) in both marine and freshwaters. Recent studies indicate that many components of the DON pool can play an active role in supplying, directly or indirectly, nitrogen nutrition to phytoplankton and bacteria and may affect community metabolic rates and species composition of the ambient microbial assemblage. Elucidate the role of DON in the development and extend of eutrophication-driven hypoxia is essential to set successful management strategies in order to reduce hypoxia development and recover ecosystems experiencing hypoxia at present, to better protect marine biodiversity.

Summary Description of Project

The main objective of the DON-Hypo project was to evaluate the role of DON in planktonic and bacterial community metabolic rates in the Baltic Sea (the largest coastal area suffering from eutrophication-driven hypoxia) and to evaluate the effects of global warming on the interaction between DON inputs, metabolic rates and oxygen dynamics.

We have contributed to the main goal of the project by:

1) Determining the DON content of a selected coastal station in the Baltic Sea 2) Determining the DON inputs from different sources to estimate the availability and reactivity of DON from various sources (municipal wastewater treatment plant effluents, atmospheric deposition, rivers) 3) Assessing the metabolic rates responses from both whole planktonic and bacterial coastal marine communities to DON additions from different sources 4) Assessing metabolic rates responses from both whole planktonic and bacterial coastal marine communities to the combined effect of DON inputs and warming 5) Assessing the chemical composition of DON from different sources and from different seasons on coastal waters, as well as biological utilization of specific compounds after 7-days incubations
We have sampled monthly 3 selected rivers during one year (Lyckebyån, Emån and Umeälven) at different latitudes of Sweden and representative of different land-use patterns. The sampling effort has been higher than previously proposed by the project, as it has been coordinated with a wider sampling of the selected rivers. Atmospheric deposition and water from a municipal wastewater treatment plant have been sampled 4 times each since the beginning of the project. Results derived from this sampling will be submitted in the near future to the open journal Biogeosciences.
Eight experiments to assess metabolic rates responses to DON additions have been performed in August 2012, January 2013, April 2013, May-June 2013, July 2013, August 2013, November 2013 and December 2013. Results from one of these experiments have been presented at the ASLO 2013 Aquatic Sciences Meeting. Additionally, results from 2 experiments have been presented in a seminar at the Aquatic Ecology department (Lund University).
Two experiments to assess metabolic rates responses to combined effect of warming and DON additions have been performed in summer (September 2013) and winter (March 2014). Results from one of these experiments have been presented as a poster in the Ocean Sciences meeting 2014, held on Honolulu on February 2014.
We have implemented diverse water analysis, such as Dissolved Primary Amines (DPA) analysis, Total Dissolved Nitrogen (TDN), ammonium (NH4), Bacterial production, Chlorophyll a content and inorganic nutrients analysis with an automated analyser. All samples analyses from experiments and sources have been performed: a total of 738 samples have been analysed for DPA, TDN, inorganic nutrients and ammonium; 666 for chlorophyll a content and 250 for BP. We have also implemented a Matlab program to calculate metabolic rates from oxygen data measured continuously with Pre-Sens oxygen sensors.

Description of the main S&T results

The main results obtained with the present project are:

-Monthly data over a year on dissolved organic nitrogen, inorganic nutrients, TDN, DPA, ammonium, chlorophyll a concentration, and flow from 3 different Swedish rivers representative from different land-use patterns and different latitudes. These results will be submitted to the open-access peer-review scientific journal Biogeosciences in a short future.

-Metabolic rates of whole planktonic and bacterial communities for a total of 8 experiments with 5 different treatments performed on duplicates. Data for TDN, DPA, Gross Primary Production (GPP), Net Community Production (NCP) and Community Respiration (CR), DON, Bacterial Production (BP), inorganic nutrients, pigment composition, ammonium, dissolved organic carbon (DOC) and chlorophyll a have been analysed in duplicate for all experimental bottles.

-CR and BP, as well as TDN, DPA, DON, inorganic nutrients, DOC, ammonium for a 2 warming experiments conducted on the dark.

-Activation energies and Q10 values for community respiration and gross primary production derived from the relationship between metabolic rates and temperature for all the experiments.

-A total of 55 samples have been analysed to determine the specific chemical composition of sources of DON to coastal waters, as well as in marine coastal water and at the beginning and end of the experiments, to assess bioavailability of DON.

Figures representing these results are provided as attached document.

The main conclusions obtained from the above-mentioned results are:

The results obtained with the present project support the prediction that warming of the Baltic Sea will increase planktonic respiration rates faster than planktonic primary production. Higher increases in respiration rates than in production may lead to depletion of the oxygen pool, further aggravating hypoxia in the Baltic Sea.

DON inputs have been found to have a synergistic effect with temperature, further increasing respiration rates with warming.

Nutrient reduction plans should also include the organic fraction of the nutrients, specifically DON and have into account the synergistic effects with warming.

Potential impact

Predictions on effects of global warming and eutrophication in dissolved oxygen dynamics in near and far future are needed to elucidate the consequences of the lack of oxygen on biodiversity and ecosystems structure and functioning. The results of this project could help better protect marine ecosystems, preserve biodiversity and set successful management targets for coastal waters.

Results obtained here will be published in peer-review scientific journals and a press release will be made to inform citizens and policy makers on the importance of taking into account the organic fraction of the nutrients when designing nutrient reduction plans aimed to reduce eutrophication-driven hypoxia on coastal areas, specially in the Baltic Sea.