Size diversity as an expression of phytoplankton community structure and the identification of its patterns on the scale of fjords and channels

Large microalgae, such as diatoms can generate high levels of biomass, unlike small phytoplankton components in the austral fjords and estuarine systems of Chile. We propose a cell-size-based diversity index (H′sizeH′size) measured as chlorophyll-a concentrations (Chl-a) to determine if the relationship between H′sizeH′size versus Chl-a results in a hump-shaped distribution considering: (i) the intermediate disturbance hypothesis, (ii) phytoplankton size colonization strategies, and (iii) predictions of low H′sizeH′size with high or low Chl-a levels and peaks in H′sizeH′size with intermediate Chl-a levels. The functional responses of phytoplankton are tightly coupled to environmental conditions. Could, then, the relationship between H′sizeH′size and Chl-a occur on a particular temporal/spatial scale? Herein, we analyze data from three CIMAR-Fiordos cruises performed between 2001 and 2002 within areas of different continental influence from 41° to 47°S. Phytoplankton samples were taken at different locations and depths, filtered to obtain Chl-a with and without size-fractionation (>20 μm, <11 μm), and then used to calculate H′sizeH′size. Total Chl-a (TChl-a) changed by two orders of magnitude between different cruises and areas. The small phytoplankton fraction (<11 μm) dominated with low TChl-a values (≤1 mg m–3), as did the large phytoplankton fraction (>20 μm) with high TChl-a values (≥1 mg m–3). Although other relationships can be found on smaller scales, we determined unimodal (hump-shaped) relationships through quadratic quantile regressions between the H′sizeH′size index and TChl-a and between H′sizeH′size or TChl-a and the abiotic factors when considering data from all three cruises and areas. H′sizeH′size versus TChl-a was driven by one variable or by different combinations of variables, and according to low–high disturbance level: high TChl-a and larger size classes predominated in deeper upper mixed layers having higher light availability, whereas shallower upper mixed layers with lower light availability presented lower TChl-a and predominantly smaller cell size classes. Thus, the larger temporal/spatial scales used here allowed us to test the hypothesis of the unimodal expression of phytoplankton patterns in these coastal ecosystems.