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Carbon dioxide variability in the Gulf of Trieste (GOT) in the Northern Adriatic Sea

Final Report Summary - CO2 GULF OF TRIESTE (Carbon dioxide variability in the Gulf of Trieste (GOT) in the Northern Adriatic Sea)

Ocean margins can be either a sink or a source for atmospheric CO2 and understanding this source / sink term and controlling mechanisms for air-sea carbon dioxide fluxes in coastal waters may substantially alter the view of the global carbon budget for land and oceans.

The Gulf of Trieste (GOT) is a semi-enclosed basin situated in the northern part of Adriatic Sea. It is largely affected by riverine inputs that provide the basin with freshwater and land derived nutrients and by large changes of air-sea exchange during Bora high wind events. The northern Adriatic is also one of the most productive regions in the Mediterranean. The unique combination of these environmental processes and relatively small area makes the region an ideal study site for investigations of air-sea interaction and relationships between biology and carbon chemistry. The main goals of our proposal were to collect and utilise the first measurements of CO2 flux in the GOT to address the following objectives:

- Determine whether the GOT is a sink or a source for atmospheric CO2. To study the temporal and spatial variability of air-sea CO2 fluxes.
- Quantify the biological and physical processes that control air-sea carbon dynamics in coastal waters of the GOT over ranges of time scales. Specifically, considered the effect of extreme fresh water input, eutrophication, phytoplankton blooms, and bora wind events.

Progress toward objectives:

With only one year of funding, our objectives have been achieved and most proposed hypothesis have been tested. The results are published in highly ranked peer-reviewed journals and are briefly summarised below:

O1: Our measurements (Turk et al., 2010a), collected during 2007 and 2008 show that the GOT is a net annual sink of atmospheric CO2 of - 2 - 3 mol m-2 yr-1. With the exception of summer 2008, surface waters of the GOT were undersaturated with respect to atmospheric pCO2. The seasonal pCO2 cycle is characterised by a summer maximum and spring and fall minima. The seasonal and interannual variability is dominated by heating and cooling based on strong correlations with sea surface temperature. These correlations break down during convective mixing in the fall, bora wind events, and brief periods of freshwater input.

O2: The presence of river plumes entering the GOT can cause significant short-term decreases in sea surface pCO2 (about 100 uatm) due to the mixing of low dissolved inorganic carbon (DIC) river water with higher DIC and saline GOT water and biological activity associated with increased freshwater nutrient input (Turk et al., 2010a). Bora wind events may play a significant role in the short-term variability of pCO2 in the GOT, possibly due to the mixing and advection of water masses with lower SSS (sea-surface salinity) and pCO2 (Turk et al., 2011). Three representative case studies on the effect of bora wind episodes on surface water pCO2 show sudden increase of pCO2 by 30 - 50 uatm a few hours after the onset of the bora, regardless of pre-bora water column conditions. Bora wind increased the flux of CO2 from the atmosphere into the sea on an annual time scale by more than 30 %.

Broader impact:

This project provided the first high-frequency measurements of water pCO2 in the northern Adriatic and new insides on carbon dynamics in the GOT. Results were published in high impact journals, presented in conferences and in media. Institutions in the Mediterranean are following the lead initiated in this study, which will impact the assessment of the atmospheric CO2 budget in coastal areas worldwide. The study has shown that coastal estimates of CO2 flux through the air-sea boundary will have to be complemented with simultaneous measurements of pCO2 in the atmosphere and pCO2 in the water at different depths near the sea-surface. Environmental agencies of countries that border the sea will include findings in their estimates of a CO2 budget.