Maren Moltke Lyngsgaard:
Vertical distribution of pelagic photosynthesis – implications for marine ecosystem dynamics

As phytoplankton photosynthesis is dependent on light, one might assume that all the phytoplankton activity occurs in the surface of our oceans. This assumption was, however, challenged early in the history of biological oceanography when chlorophyll sampling and fluorescence profiling showed deep chlorophyll maxima (DCM) to be a general feature in the ocean. Today, it is generally accepted that DCMs occur in most of our oceans still, despite this empirical knowledge, subsurface primary production is still largely ignored in marine science.

The work included in this PhD examines the vertical distribution of phytoplankton activity and its importance to several major themes of societal interest; carbon transport, fisheries ecology and eutrophication effects. The main focus of the thesis is on the importance of vertical distribution of primary production to ecosystem functioning and how this vertical distribution pattern is related to nitrogen loading in order to elucidate potential trajectories in the recovery from eutrophication. In addition, the importance of vertical phytoplankton activity is evaluated in an oligotrophic tropical ocean as well as in a North Atlantic spring bloom in order to gain knowledge on region-specific importance of subsurface phytoplankton activity.

The major part of the PhD work was related to the recalculation and analysis of a 15 year long dataset (from 1998 to 2012) of photosynthetic parameters, phytoplankton biomass, chlorophyll, oxygen, nutrients and physical parameters such as salinity, temperature and light. The dataset includes six different stations located in the Baltic Sea transition zone made available by the Danish National Aquatic Monitoring and Assessment Program (DNAMAP). General patterns were derived from this dataset of vertical and seasonal variation in primary production, and nitrogen loading was then related to these patterns. The opportunity to combine long term data with detailed process studies became available through a two week field campaign on one of the stations from the survey programme which was conducted in July 2010. The purpose of the field study was to examine the importance of the vertical phytoplankton activity for the ecosystem functioning in further detail.

Before the initiation of the PhD work I measured, among many variables, primary production and variable fluorescence in the Sargasso Sea during my four month participation in the Danish circumnavigating research expedition Galathea III (from year 2006-2007). This data was, during this PhD, analysed to elucidate underlying mechanisms for eel larvae distributions. In May 2008, I spent one month measuring variable fluorescence in the North Atlantic on the RV KNORR as a part of the North Atlantic Bloom Experiment (NABE). These data were used in the PhD study to describe the importance of the vertical distribution of phytoplankton and their activity for the carbon transport related to the North Atlantic spring bloom. The five manuscripts included in the study describe the main findings related to the analysis of these datasets.

In manuscript I, the recalculation of primary production from the survey programme was used to quantify deep primary production (DPP). These vertical distribution patterns of primary production were then related to nitrogen loading. The effect of reduced nutrient loading to the Danish coastal waters on primary production (PP) has been widely discussed. It is shown here that not only is nitrogen loading significantly related to total water column PP, but it is even more strongly related to the vertical distribution of PP. This relationship suggests that reduced loading will increase the deep primary production both in magnitude but also relative to the total water column PP which is suggested to have a positive effect on oxygen conditions in the bottom waters.

Manuscript II goes into further detail with the prediction of increased deep primary production suggested in manuscript I. Here, the results of the field work carried out in Aarhus Bight were used to examine the characteristics of DPP versus PP in the surface layer, assuming that the water column exhibits different environments with respect to light and nutrients for phytoplankton activity. The potential for changes in oxygen production has, previously been ignored in consideration of the oxygen conditions in the bottom waters of this region. However, this study suggests that DPP, in addition to physical processes has a positive effect on the oxygen concentration in the bottom waters. Furthermore, the photosynthetic characteristics and species composition of the subsurface phytoplankton community showed signs of an active community generally comprised of larger cells (mainly dinoflagellates) than in surface waters. The sedimentation of organic material related to the subsurface phytoplankton community was enhanced compared to sedimentation in surface waters. The process studies in Aarhus Bight overall suggest that reduced nitrogen loading will change ecosystem functioning in a manner likely to reduce the likelihood and/or intensity of hypoxia events.

In manuscript III, the DNAMAP dataset was examined for general distribution patterns in primary production, chlorophyll, nutrients and zooplankton biomass. Global estimates of PP are often based on chlorophyll derived from satellite imagery. Chlorophyll is, however, constantly varying with nutrients and light. Hence, chlorophyll concentration is not a robust indicator of the carbon flow within a pelagic system. This study clearly shows that PP and chlorophyll are decoupled during summer time which was mainly attributed to the seasonal variation in the carbon to chlorophyll ratio. In the search for a better proxy for carbon dynamics than chlorophyll concentrations, a parameterisation of the seasonal variation in primary production was developed. The seasonal variation of chlorophyll was clearly different from that of carbon biomass, especially with respect to the magnitude of the spring bloom. This suggests that the spring bloom may have received far more attention than it deserves, as chlorophyll rather than carbon biomass has most often been used to describe phytoplankton biomass.

In manuscript IV, physical and biological parameters were used to elucidate the mystery of eel larvae distribution in the Sargasso Sea. It has long been known that eels spawn in the Sargasso Sea but the mechanisms behind the concentrated eel larvae in frontal systems has not yet been described, since there has been no apparent link between biomass or PP and the larvae. In this study, however, it was shown that localised vertical mixing could be related to the physiological state of the phytoplankton in these deep frontal zones, potentially making those a better source of nutrients to support the planktonic food web. This study overall emphasizes the importance of the deep primary production dynamics and phytoplankton physiology to the higher trophic levels and carbon flow in the Sargasso Sea.

Manuscript V provides another example of the relevance of understanding phytoplankton distributions and activity below the surface in order to understand the dynamics of the system as a whole. The carbon flow related to the North Atlantic spring bloom has been studied intensively and it has been assumed that the phytoplankton species occurring in the surface waters are a good indicator of the species likely to contribute to vertical carbon transport. The study presented here, however, showed the presence of resting spores of the diatom species Chaetoceros aff. diadema at great depths. The organic material found in the sediment traps containing C. diadema resting spores was sinking very fast and contributing disproportionally to the carbon export to deep waters. Overall, this study suggests that the vertical distribution of phytoplankton species and activity is important to the carbon flow in our oceans.

The results of this PhD work show that the vertical distribution of phytoplankton and their activity is important for the understanding, dynamics and functioning of pelagic ecosystems. It, thus, emphasizes that future research and modelling exercises aimed at improving understanding of pelagic ecosystems and their role in the global ocean should include a consideration of the vertical heterogeneity in phytoplankton distributions and activity.