Erik Askov Mousing:
Macroecological patterns in the distribution of marine phytoplankton

Date: 15-10-2013    Supervisor: Katherine Richardson & Marianne Ellegaard

Marine phytoplankton are responsible for approximately half of the global total primary production. The photosynthesis they carry out sustains higher trophic levels in the marine ecosystem. Changes in phytoplankton community composition can have cascading effects on food web dynamics, total production, biochemical cycling and have a direct impact on the global carbon cycle through the biological pump. Understanding the processes controlling phytoplankton primary production and community composition at the global scale and how these interact with climate change are, therefore, imperative for predicting future ocean function.

This PhD thesis investigates and describes macroecological patterns in the distribution and diversity of marine phytoplankton. The primary focus has been to describe macroecological patterns in phytoplankton total biomass and community structure and relate these patterns to in situ abiotic conditions (primarily temperature, salinity, mixed layer dynamics and ambient nutrient concentrations) in order to elucidate the primary bottom-up processes that control phytoplankton communities.

In order to do this, I investigate and present several data sets that have been assembled over both spatial and temporal scales. Macroecological patterns are investigated on the global scale through a data set collected on the circumnavigating Galathea 3 expedition in 2006- 2007. Specific hypotheses on phytoplankton size, silica recycling and bloom dynamics are investigated using more confined datasets collected in the North Atlantic in 2008, in the Disko Bay, Greenland in 2007 and in the Black Sea in 2007. Several statistical techniques are used but the focal point resolves around statistical modeling and multi-model inference using an information-theoretic approach. The results are presented as five research chapters shaped as manuscripts.

In Manuscript I, the macroecological patterns in phytoplankton community size structure were investigated in relation to temperature and inorganic nutrient concentrations. Although temperature has been shown to directly affect phytoplankton size at small scales, it has been largely unknown to what degree this effect scales up to the global community. Here, it is shown that temperature has an important and universal direct effect as well as an indirect effect on phytoplankton community size structure (possibly through the development of thermal stratification limiting the flux of nutrients from the deep ocean). This affect has important implications for the global carbon cycle and should be included in future climate models.

In manuscript II, changes in the mean cyst size of dinoflagellates are investigated in relation to temperature changes during the Little Ice Age and the Medieval Climate Anomaly. It is shown that cysts of both individual dinoflagellate species and the community as a whole decrease with increasing temperature and it is argued that cyst sizes, measured at the intraspecific scale, show great potential in elucidating the effect of changing temperature through time.

In manuscript III, the macroecological distribution of phytoplankton biomass and community size structure are investigated in relation to ambient inorganic macronutrient concentrations. Although nutrients are known to control both biomass and growth of phytoplankton from local to the global scale, their relative importance in relation to one another and the importance of interactions between several nutrients are unknown for the open ocean on the global scale. Here, it is shown that all of the measured nutrients contribute to the control of phytoplankton biomass and community size structure at the global scale. The statistical approach applied indicated that different nutrients were, apparently, co-limiting the examined phytoplankton processes in different regions of the world’s oceans. It is also shown that there is a strong increase in both phytoplankton total biomass and the fraction of large phytoplankton in the community at N/P ratios between 8 and 16. Thus, it is argued that nutrient ratios play an important role in both yield and rate limitation of phytoplankton. In manuscript IV, the dissolution state of silicate over time in the Black Sea is investigated using protist remains in a sediment core representing the last 100 years and the results used to propose a new hypothesis concerning silica recycling in this regional sea. Diatoms in the Black Sea have been believed to be silica limited due to human induced changes in nutrient input beginning in the 1970s. However, increasing silicate in the deep ocean over the same period has indicated that there is an overlooked source of silicate and has brought the paradigm of silica limitation into question. Here, it is shown that silicate-using protists became more diluted in the sediment after 1970 and argue that the unknown silica source is probably increased silica recycling caused by increasing annual production due to increasing nitrogen concentrations.

Manuscript V investigates changes in phytoplankton community composition during the North Atlantic spring bloom in 2008 and relates these changes to ambient physical conditions. Although the general seasonal phytoplankton succession pattern in this region has been identified as going from diatoms to dinoflagellate to flagellates, almost nothing is known about the factors controlling small scale spatial and temporal heterogeneity in phytoplankton species composition during the spring bloom, itself. Here, two different phytoplankton communities were identified, each associated with specific hydrographic features. The major difference between the two groups was the presence/absence of large Chaetoceros species and, where present, these species dominated the diatom biomass. The availability of inorganic nutrients did not appear to be important in establishing the overall phytoplankton distribution patterns recorded here. However, changes in the biomass of Chaetoceros at the shallowest sampling depths correlated with wind-induced turbulence.

Together, the studies presented here indicate that macroecological investigation of phytoplankton abundance and species distributions has tremendous potential in terms of developing understanding of fundamental processes relating to carbon uptake by photosynthesis in the ocean. Understanding these fundamental processes is a prerequisite for predicting how changing ocean conditions may influence phytoplankton communities and their role in global carbon cycling.