Marine phytoplankton size–frequency distributions: Spatial patterns and decoding mechanisms

Body size–frequency distributions are structural components of guilds and communities which relevance in phytoplankton guilds is increasingly recognized. Here, we describe in a coastal marine environment (southern Adriatic/Ionian Sea, south-eastern Italy) the seasonal patterns of variation of nano/microphytoplankton size–frequency distributions on three spatial dimensions (depth, distance from the coast and along-shore position). The aims of this study were: (1) to quantify the relative contribution of the spatial components to the overall variation of the phytoplankton size–frequency distributions; (2) to analyse the influence of two main forcing factors (i.e., water column stability and nutrient concentration) on phytoplankton size–frequency patterns; and (3) to evaluate the mechanistic basis of the observed size patterns. In accordance with the last aim, we performed a comparative analysis of the patterns of variation of both size–frequency distributions and size fractions, in order to cover two scales of observation characterised by different grain and extent. The study was performed at 35 stations located on seven coast-open sea transects. At each station, phytoplankton size–class frequency distributions, total and fractionated phytoplankton biomasses and nutrient concentrations were determined at three depths, while profiles of temperature and salinity, from which water column stability was calculated, were drawn for the whole water column. Significant patterns of phytoplankton size–frequency distribution were observed along the spatial gradients and distance from the coast was the most significant source of spatial variation in phytoplankton size–frequency distribution. Along the distance gradient, opposite trend was observed at the two scales of observation. At level of size classes, relatively large size classes increased with increasing distance from the coast; accordingly, statistical descriptors of the size– frequency distributions had spatially explicit distributions, being more skewed and narrower within the first 3 Nautical Miles (NM) from the coast. At level of size fractions, the relative importance of microphytoplankton decreased with increasing distance from the coast. It is argued that the observed spatial patterns of phytoplankton size structure result from the combination of the hierarchical interaction of water column stability with nutrient continental input determining tri-dimensional nutrient gradients and niche filtering of phytoplankton size classes on these gradients through abiotic and biotic filters.