Abstract
Despite ample evidence of micro- and small-scale (i.e., millimeter- to meter-scale) phytoplankton and zooplankton patchiness in the ocean, direct observations of nutrient distributions and the ecological importance of this phenomenon are still relatively scarce. In this context, we first describe a simple procedure to continuously sample nutrients in surface waters, and subsequently provide evidence of the existence of microscale distribution of ammonium in the ocean. We further show that ammonium is never homogeneously distributed, even under very high conditions of turbulence. Instead, turbulence intensity appears to control nutrient patchiness, with a more homogeneous or a more heterogeneous distribution observed under high and low turbulence intensities, respectively, under the same concentration in nutrient. Based on a modelling procedure taking into account the stochastic properties of intermittent nutrient distributions and observations carried out on natural phytoplankton communities, we introduce and verify the hypothesis that under nutrient limitation, the “turbulent history” of phytoplankton cells, i.e., the turbulent conditions they experienced in their natural environments, conditions their efficiency to uptake ephemeral inorganic nitrogen patches of different concentrations. Specifically, phytoplankton cells exposed to high turbulence intensities (i.e., more homogeneous nutrient distribution) were more efficient to uptake high concentration nitrogen pulses (2 µM). In contrast, under low turbulence conditions (i.e., more heterogeneous nutrient distribution), uptake rates were higher for low concentration nitrogen pulses (0.5 µM). These results suggest that under nutrient limitation, natural phytoplankton populations respond to high turbulence intensities through a decrease in affinity for nutrients and an increase in their transport rate, and vice versa.
Highlights
Information on the qualitative and quantitative nature of micro-scale nutrient distribution is critically needed to bridge the gap between bacteria and phytoplankton and higher trophic levels to improve our general understanding of structures and functions in marine systems
While the resolution of this specific issue is beyond the scope of this preliminary study, Under nutrient limitation, phytoplankton cells exposed to high turbulence intensities would this observation suggests that turbulence history may play a more critical role than the actual density experience a low background of evenly distributed nutrients and would exhibit low affinity for of the nutrient patches encountered by phytoplankton cells in conditioning their surge uptake abilities
These results indicate that turbulence controls the microscale distribution of ammonium, switching from a more homogeneous to a more heterogeneous distribution respectively under high and low turbulence intensities for the same bulk concentration
Summary
Investigations of micro- to small-scale (typically millimeter- to meter-scale) distributions of viruses, bacteria, phytoplankton, and zooplankton populations revealed their patchy character, see e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22], in particular in relation with turbulence [23,24,25]. This hypothesis holds true under the general assumption that the parameters of the Michaelis-Menten kinetics remain constant irrespective of ambient nutrient concentration [39] It becomes, fundamentally unrealistic given the well-known abilities of nutritionally limited phytoplankton cells to enhance their uptake of nutrients in the presence of ephemeral point source [40,41,42]. Fundamentally unrealistic given the well-known abilities of nutritionally limited phytoplankton cells to enhance their uptake of nutrients in the presence of ephemeral point source [40,41,42] In this context, the first aim of this work is to briefly rehearse the description of a simple technique allowing the continuous sampling of nutrient from surface waters [28], to critically assess its potential limitations, and to illustrate its validity to characterize nutrient patchiness in the specific framework of sampling experiments conducted in the eastern English Channel. We validate our mechanistic hypotheses through a designed field experiment devoted to assess the surge uptake rates of natural phytoplankton communities under ammonium limitations when exposed to ammonium pulses of low and high concentrations
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