Abstract
Species composition plays a key role in ecosystem functioning. Theoretical, experimental and field studies show positive effects of biodiversity on ecosystem processes. However, this link can differ between taxonomic and functional diversity components and also across trophic levels. These relationships have been hardly studied in planktonic communities of coastal upwelling systems. Using a 28-year time series of phytoplankton and zooplankton assemblages, we examined the effects of phytoplankton diversity on resource use efficiency (RUE, ratio of biomass to limiting resource) at the two trophic levels in the Galician upwelling system (NW Iberian peninsula). By fitting generalized least square models, we show that phytoplankton diversity was the best predictor for RUE across planktonic trophic levels. This link varied depending on the biodiversity component considered: while the effect of phytoplankton richness on RUE was positive for phytoplankton RUE and negative for zooplankton RUE, phytoplankton evenness effect was negative for phytoplankton RUE and positive for zooplankton RUE. Overall, taxonomic diversity had higher explanatory power than functional diversity, and variability in phytoplankton and zooplankton RUE decreased with increasing phytoplankton taxonomic diversity. Phytoplankton used resources more efficiently in warmer waters and at greater upwelling intensity, although these effects were not as strong as those for biodiversity. These results suggest that phytoplankton species numbers in highly dynamic upwelling systems are important for maintaining the planktonic biomass production leading us to hypothesize the relevance of complementarity effects. However, we further postulate that a selection effect may operate also because assemblages with low evenness were dominated by diatoms with specific functional traits increasing their ability to exploit resources more efficiently.
Highlights
Marine phytoplankton is responsible for roughly half of the global primary production (50 Pg C year−1; Chavez et al, 2011), contributes to nutrient cycling and regulation of climate dynamics, affects the fate of adjacent trophic levels (Richardson and Schoeman, 2004), and, constrains fishery catches (Chassot et al, 2010)
We found lower RUEzp when phytoplankton biomass was dominated by diatoms, which could be likely explained by several factors including the lower impact that mesozooplankton compared to microzooplankton has on phytoplankton (e.g., Fileman and Burkill, 2001), the inhibition that diatom exudates can exert on zooplankton grazing (e.g., FIGURE 6 | Scatterplots showing the relationships between RUEpp (A–C) and RUEzp (D–F) with CWMsilica (A,D), CWMmotility (B,E), and CWMchain (C,F)
We found that taxonomic and functional diversity enhance planktonic resource use efficiency in a coastal upwelling system
Summary
Marine phytoplankton is responsible for roughly half of the global primary production (50 Pg C year−1; Chavez et al, 2011), contributes to nutrient cycling and regulation of climate dynamics, affects the fate of adjacent trophic levels (Richardson and Schoeman, 2004), and, constrains fishery catches (Chassot et al, 2010). Marine phytoplankton is an extremely diverse group of organisms (De Vargas et al, 2015), and this diversity, encompassing a large variety of Phytoplankton Effect on RUE in a Coastal Upwelling life histories, is an essential factor that affects the whole structure of marine ecosystems (Naeem, 2012). It is the diversity of functional traits that differs among and within species and taxonomic groups, the key component determining the fitness of planktonic communities along environmental gradients and influencing the functioning of pelagic ecosystems (Irwin and Finkel, 2018). Other authors did not find a relationship between phytoplankton species richness and ecosystem productivity (e.g., Cermeño et al, 2013)
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