Cycling of dissolved and particulate organic matter in the ocean west of the Antarctic Peninsula
Dissolved organic matter (DOM) in the oceans stores as much carbon as the atmosphere for thousands of years. However, our understanding of production, transformation and removal processes of DOM is still incomplete. At the West Antarctic Peninsula (WAP), rapid warming led to increased atmospheric and oceanic temperatures during the second half of the 20th century with reduced sea-ice cover and increased glacial melting. The WAP supports a productive ecosystem with intense primary production during the austral spring and summer when solar radiation is high and sea ice cover is reduced. Research on dissolved organic matter in this region is scarce. Concentrations of DOM here are low compared to lower latitudes but reasons for this remain unclear and the cycling of DOM is not fully understood. Because of the recent climate change in this region, its geographical distance from anthropogenic sources and the distinct seasonality of the ecosystem’s productivity, the WAP represents an ideal location to study processes involved in autochthonous DOM dynamics. This thesis integrates a suite of biogeochemical and physical data to develop an understanding of dissolved organic carbon (DOC) and nitrogen (DON) cycling at the WAP. Samples have been collected for spatial analysis with the U.S. led Palmer Longterm Ecological Research Program (PAL LTER) cruise team in 2017 and samples for temporal analysis are available from the UK’s Rothera Research Station as part of the Rothera Time Series (RaTS) from 2013 to 2016. In combination with other available physical, biogeochemical and biological data, processes driving the distribution and cycling of DOM over a range of spatial and temporal timescales are investigated. The temporal analysis from the RaTS data found DOC production occurring alongside particulate organic carbon production contrasting earlier studies where DOM production was found to occur later with a time lag of a few days to weeks. This thesis shows that DOC is produced and released directly by phytoplankton while DON shows more variable results. This might be due to high rates of DON cycling by both bacteria and phytoplankton. The spatial analysis (PAL LTER) confirmed earlier studies showing low concentrations of dissolved organic carbon and nitrogen. There is more variability and slightly higher concentrations of DOM in coastal waters compared to offshore regions. This is potentially due to higher primary production and bacterial responses but could also be affected by the introduction of glacial meltwater. DON correlates well with bacterial activity while DOC can be related to either bacterial or phytoplankton activity showing the different mechanisms affecting both DOC and DON production and removal. At stations with high bacterial activity in the surface waters, DOC and DON concentrations were found to be high but decrease rapidly with increasing depth. Due to a temporal offset in the retreat of sea ice from the open ocean towards the shore, the sampled stations are found to be at different stages of the phytoplankton bloom which is reflected in the biogeochemical data including DOC and DON concentrations. Particulate and dissolved organic matter cycling is coupled to some extent. DOC appears to be produced during the development of the first phytoplankton bloom of each season but is decoupled from direct production of POC thereafter, possibly due to bacterial removal and production processes. DOC and DON are highly decoupled throughout the investigated seasons and across the WAP shelf. The C and N isotopic compositions of particulate organic matter in both the spatial and the temporal data sets confirm intense upper-ocean recycling of organic matter with little export to greater depths. Further, the N-isotopic composition shows that nitrification plays an important role in the upper ocean at the WAP with nitrified nitrate and potentially ammonium being produced and taken up by phytoplankton at the later stage of phytoplankton activity. Ammonium measurements were only available for the RaTS data sets but show that the seasonal variability is intense. Increased production of ammonium in the upper ocean is related to lowered DON concentrations showing rapid ammonification. The contribution by meltwater from both glaciers and sea ice was analysed. While direct contributions of DOM from these sources are likely, they are suggested to be minor due to intense dilution with seawater. However, indirectly, DOM dynamics are likely affected intensely by the addition of sea-ice algae, bacteria, particulate organic matter and nutrients and effects on the physical structure of the water column, all of which can affect the production, transformation and removal of DOM. This thesis shows that processes driving DOC and DON dynamics are complex in the ocean of the WAP. There are different processes acting on DOM compounds in different regions of the WAP at different timescales. DOM produced at the WAP seems to be of a highly labile nature, supported by low DOC:DON ratios overall. High surface DOM concentrations decreased rapidly with depth which shows high rates of bacterial degradation. These findings suggest that if DOM production increases in this region, as projected by various studies due to a warming climate and increased meltwater addition, upper-ocean cycling of carbon and nitrogen might increase while carbon export decreases. This thesis contributes to our understanding of carbon and nitrogen cycling in high productivity Southern Ocean shelf environments with implications for the functioning of the regional biological carbon pump.