Abstract
Understanding the mechanisms of phytoplankton community assembly is a fundamental issue of aquatic ecology. Here, we use field data from transitional (e.g. coastal lagoons) and coastal water environments to decode patterns of phytoplankton size distribution into organization and adaptive mechanisms. Transitional waters are characterized by higher resource availability and shallower well-mixed water column than coastal marine environments. Differences in physico-chemical regime between the two environments have been hypothesized to exert contrasting selective pressures on phytoplankton cell morphology (size and shape). We tested the hypothesis focusing on resource availability (nutrients and light) and mixed layer depth as ecological axes that define ecological niches of phytoplankton. We report fundamental differences in size distributions of marine and freshwater diatoms, with transitional water phytoplankton significantly smaller and with higher surface to volume ratio than marine species. Here, we hypothesize that mixing condition affecting size-dependent sinking may drive phytoplankton size and shape distributions. The interplay between shallow mixed layer depth and frequent and complete mixing of transitional waters may likely increase the competitive advantage of small phytoplankton limiting large cell fitness. The nutrient regime appears to explain the size distribution within both marine and transitional water environments, while it seem does not explain the pattern observed across the two environments. In addition, difference in light availability across the two environments appear do not explain the occurrence of asymmetric size distribution at each hierarchical level. We hypothesize that such competitive equilibria and adaptive strategies in resource exploitation may drive by organism’s behavior which exploring patch resources in transitional and marine phytoplankton communities.
Highlights
The underlying mechanisms driving species coexistence have long been the subject of intense debate [1, 2]
The coastal and transitional water ecosystems sampled in this study showed a wide range of physical-chemical and morphological characteristics that reflected a broad range of environmental factors that affect phytoplankton cell size
All parameters considered in this work, were more variable in transitional than coastal waters considering the standard error as descriptor of environmental variability (Table 1)
Summary
The underlying mechanisms driving species coexistence have long been the subject of intense debate [1, 2]. Decoding Size Distribution of Phytoplankton Communities mechanisms, accounting for niche partitioning [4], such as environmental fluctuation [3, 5, 6], temporal succession [7], trophic [8] or chaotic population dynamics [9] have been proposed to explain species diversity. These niche-based mechanisms can promote coexistence and shape community size structure [10]. A combination of nutrient storage capability and sinking behaviour of cells in response to different nutrient regime and mixed layer depth has been proposed to explain these patterns [26, 27]
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