719 / 2024-09-19 08:31:26

Shifting from inventory-based monitoring towards process-oriented time-series studies

time-series,nitrous oxide,water column inventory,production dynamics,isotope

Marine biogeochemical studies rely in part on regular observation having data points recorded at consistent intervals over a period of time. Practical considerations of sampling and analytical power limit such observations to just one particular spot, and inventory-based parameters obtained by in-situ sensor detection (such as CTD package) or automatic measurements (such as major nutrients, dissolved gases) are archived and published with high confidence. Nevertheless, previous efforts in collecting these marine inventories have contributed significantly to the community understandings in marine biogeochemical dynamics and human-climate interactions. It is gradually becoming difficult to analyze these increasing datasets, particularly for their lack of indicative power to reveal biogeochemical processes, such as nitrogen cycling processes involving microbial uptake, transfer and loss of reactive nitrogen. With advancing analytical capabilities in isotopic tracing, a shifted paradigm from traditional, inventory-based sampling campaigns towards process-oriented time-series studies could be a useful pathway for future marine researchers.

Recent progress in the subtropical Pearl River Estuary serves as an example that nitrous oxide dynamics may be indicated differently using monthly concentration data versus seasonal production rate measurements. In general, concentration indicates water column inventory that is the result of sources and sinks, whereas production rates reflect microbial activities under in-situ conditions. Several instances demonstrate that nitrous oxide concentration and microbial production seems to be loosely synchronized, e.g. low production rates in subtropical winter coincided with moderate water column concentrations. The commonly used statistical techniques producing difference in correlated environmental variables with concentration and rates, i.e. increasing concentration with decreasing discharge, and positively correlating production rates and temperature, could suggest interesting hypotheses for future validation. Estuarine production may not be as important as upstream transport in contributing to the water column inventory of nitrous oxide. The natural abundance isotopic and isotopomeric data of nitrous oxide, though useful indicators of production pathways in deep waters, may be less informative to shallow water due to intensive air-sea mixing.