Size-niche specific processes between mero- and holoplankton stabilize the continuum in size spectra of tropical estuarine and marine ecosystems

Abstract

Size spectra contain key information on the functioning of ecosystems. Yet, the dynamic processes that shape a zooplankton size spectrum composed of numerous species and larval stages are still poorly documented. Normalized biomass size spectra (NBSS) were constructed to describe the total zooplankton, decapod larvae, and other mero- and ichthyoplankton in three areas of Tamandaré, Brazil. Bimonthly samples were obtained with a 300 μm mesh net, from 2013 to 2015, and analyzed with a ZooScan device. NBSS slopes were steeper than expected (Estuary: slope = −2.45 + − 0.16, Bay: slope = −1.81 + − 0.10, Shelf: slope = −1.80 + − 0.09), which was clearly due to the extremely high abundance of calanoid copepods (mostly Acartia spp.), especially in the Estuary. Removal of non-holoplanktonic organisms did not affect the NBSS slope, confirming the paradigm that the NBSS slope is a conservative property of ecosystems. We used permutation tests to compare the numbers of gaps in the size spectrum (i.e., empty bins). This emptiness test detected highly significant effects of non-holoplanktonic organisms on discrete holoplankton size fractions, in all study areas. Copepods were generally dominating in the size range < 3 mm Feret length. In the Estuary, brachyuran crab larvae, fish eggs, and caridean shrimp zoeae were most important, as meroplanktonic contributors to discrete niches in the size spectrum. In Shelf and Bay areas, penaeid postlarvae and fish larvae were the most relevant large-sized taxa, in units of biovolume. Density-dependent negative interactions (top-down regulation and functional replacement) occurred between same-sized brachyuran zoeae and copepods. The processes that shape taxon-specific size spectra into a “gap-free” spectrum are discussed under an evolutionary size-niche perspective. Our results show that continuous power law size spectra are likely shaped by size-structured interactions, taxon-specific size niche strategies (e.g., competitive exclusion), and top-down density-dependent food web regulation processes.