Picoeukaryote distribution in relation to nitrate uptake in the oceanic nitracline

We investigated the relationship between picoeukaryote phytoplankton (< 2 mu m) and the deep layer of new production (NO3- uptake) in the nitracline of the eastern subtropical North Atlantic Ocean. Indices of NO3- uptake kinetics obtained within the lower 15 % of the euphotic zone demonstrate that subsurface NO3- uptake maxima are coincident with localised peaks in maximum uptake rates (V-max) and, crucially, with maximum picoeukaryote abundance. The mean rate of NO3- utilization at the nitracline is typically 10-fold higher than in surface waters despite much lower in situ irradiance. These observations confirm a high affinity for NO3-, most likely by the resident picoeukaryote community, and we conservatively estimate mean cellular uptake rates of between 0.27 and 1.96 fmol NO3- cell(-1) h(-1). Greater scrutiny of the taxonomic composition of the picoeukaryote group is required to further understand this deep layer of new production and its importance for nitrogen cycling and export production, given longstanding assumptions that picoplankton do not contribute directly to export fluxes.

Shark personalities? Repeatability of social network traits in a widely distributed predatory fish. Behavioral Ecology and Sociobiology, 68, 1995-2003.

Interest in animal personalities has generated a burgeoning literature on repeatability in individual traits such as boldness or exploration through time or across different contexts. Yet, repeatability can be influenced by the interactive social strategies of individuals, for example, consistent inter-individual variation in aggression is well documented. Previous work has largely focused on the social aspects of repeatability in animal behaviour by testing individuals in dyadic pairings. Under natural conditions, individuals interact in a heterogeneous polyadic network. However, the extent to which there is repeatability of social traits at this higher order network level remains unknown. Here, we provide the first empirical evidence of consistent and repeatable animal social networks. Using a model species of shark, a taxonomic group in which repeatability in behaviour has yet to be described, we repeatedly quantified the social networks of ten independent shark groups across different habitats, testing repeatability in individual network position under changing environments. To understand better the mechanisms behind repeatable social behaviour, we also explored the coupling between individual preferences for specific group sizes and social network position. We quantify repeatability in sharks by demonstrating that despite changes in aggregation measured at the group level, the social network position of individuals is consistent across treatments. Group size preferences were found to influence the social network position of individuals in small groups but less so for larger groups suggesting network structure, and thus, repeatability was driven by social preference over aggregation tendency.