PhD defence Freija Hauquier

Friday, May 27, 2016

Environmental and spatial constrains on Antarctic marine nematode distribution:

The distribution of organisms across the globe is not random, an observation that has stimulated the search for rules and explanations for the processes behind it. In general, there is a consensus that local species composition, richness and abundance are the result of processes that operate at different spatial and temporal scales. For example, species diversity and composition might reflect both local changes in environmental characteristics (food, sediment grain size, oxygen) as well as large-scale gradients in climate. Resolving distribution patterns therefore requires the integration of approaches at the crossing of (macro-) ecology and biogeography (Logue et al., 2011). Whether species occur at a certain place and time depends on a complex interplay of various factors. Dispersal and exchange of individuals between patches plays a crucial role in this process. Theoretically, the marine environment with its open continuous character and its presence of large-scale ocean currents forms a more connected system than geographically confined freshwater systems such as ponds and lakes. Long-distance travel of species in the ocean is thus more likely, both for active swimmers as well as for those species with passive pelagic dispersive stages (e.g., larvae). While this implies a possibility for developing cosmopolitan distributions, limits to such ubiquity are imposed by niche dynamics, where characteristics of local habitat patches preclude the presence of some species while favouring others. The situation becomes somewhat different for organisms living in seafloor sediments (the benthos) that lack pelagic dispersive stages and whose presence in the water column is therefore a sporadic event. In this instance, distance and dispersal limitation probably play a more active role in structuring communities at large spatial scales. Metacommunity theory (Leibold et al., 2004) forms one example of a theoretical framework that tries to disentangle the role of such niche effects and dispersal effects on distribution patterns of organisms. This concept served as a background for this thesis, which aims at resolving the relative contribution of local (i.e. species-species interactions, species-environment relationships) and regional (i.e. geographic separation, dispersal limitation) processes on contemporary community structure of marine nematodes in continental shelf locations (200 – 500 m water depth) of the Southern Ocean. The area and its biota share a remarkable history of isolation and glaciation events, and evolved subtle equilibria which are currently put to the test by imminent changes related to global warming. Nematoda or roundworms are mainly known as parasites in both plants and animals, but this study will focus on the free-living representatives of this phylum, which occupy the interstitial spaces in seafloor sediments. They are small in size (generally < 1 mm), are often the numerically dominant taxon within the meiofauna (densities of several thousands of individuals per 10 cm2 are not uncommon), and occur in high genus and species numbers in almost all aquatic habitats (Heip et al., 1985). Despite their endobenthic lifestyle and passive dispersal mode, hence presumed limited dispersal capacities, genera (and also some species) are widely spread. This meiofauna paradox (Giere, 2009) forms the topic of considerable debate among meiofauna ecologists, but has been challenged recently by insights gained through molecular advances. Throughout the chapters of this thesis, different aspects of nematode communities (i.e. abundance, diversity, community composition and distribution) were assessed in different ways, at different spatial scales, and with increasing taxonomic resolution. The first two research chapters adopted a correlative approach to analyse variation between nematode communities at a modest spatial scale of tens to a few hundreds of km, complemented by a temporal scale of a few years in Chapter 2. Sampling locations were all situated in the premises of the Antarctic Peninsula, but differed in local conditions and dynamics. In the first study, climate-induced ice-shelf collapse in the Larsen area east of the peninsula (Rack & Rott, 2004) resulted in drastic changes in light regime and primary productivity, hence food input for benthic communities. These benthic communities were studied 7 and 11 years after the initial ice-shelf collapse to investigate their response to this change from an ice-covered oligotrophic to a more productive system. Nematodes’ response to these changes over the course of four years pointed towards the importance of environmental filtering and colonisation rate in stimulating localised proliferation of one or a few opportunistic genera. Compared to other Antarctic continental shelf locations, the nematode communities in the Larsen area were very different in terms of genus composition, density and vertical distribution in the sediment. Differences in nematode assemblages between locations within the area itself could be related to a different timing of the loss of ice cover and related food input. A similar correlation between nematode communities and environmental conditions was demonstrated in the next study (Chapter 3), but at a bigger spatial scale involving areas under different oceanographic influence at both sides of the peninsula. In this case, variation in communities was mainly attributable to the efficiency of bentho-pelagic coupling processes and sea-ice dynamics (or the lack thereof). While these two studies were more in line with traditional ecological approaches of linking community composition at a local scale to environmental gradients (cf. species sorting within the metacommunity theory; Leibold et al., 2004), the next two chapters incorporated dynamics at a larger spatial extent. Sampling locations covered areas both within and beyond biogeographic zones and oceanographic current systems, and nematode assemblages were analysed using variation partitioning analysis at the level of the entire community (Chapter 4) or phylogeographic and population genetic techniques at a more detailed level for two selected genera and their species (Chapter 5). Outcomes of both studies were largely congruent and highlighted the importance of historical separation and dispersal limitation for nematode community assembly at large spatial scales. More specifically, nematode genus and species communities in Chapter 4 were largely different between the different locations, and these differences increased with increasing distance between locations. In a similar fashion, populations for several species within the genera Sabatieria and Desmodora in Chapter 5 showed high levels of genetic differentiation depending on their location. Although both results could partially be linked to changes in environmental conditions, distance and spatial heterogeneity proved to be more important drivers for the observed differences. A possible explanation could be that the current systems operating in the area are not efficient enought to maintain high levels of connectivity between nematode communities separated by several hundreds of km. The work performed during this thesis has revealed that 1) nematode communities in the Southern Ocean differ according to their geographical location as well as vertical position in the sediment, 2) genera are widely distributed but show different relative abundances between locations and sediment depth layers, 3) species have either restricted or wide distributions, 4) influence of local processes on genus and species occurrence is mainly limited to smaller spatial scale and communities at the seafloor surface, 5) regional processes (historical events, dispersal limitation) gain importance at larger spatial scales, and finally 6) cryptic species are present within one genus which demonstrates the potential bias in macroecological studies when relying on morphological species delineations alone. Together, these aspects provide information on why species are distributed in a certain way, and might help to understand and predict how community patterns of small organisms might change in the near future. Especially in light of current climate change, further assessment of species distribution patterns and structuring processes is vital.