Effect of CO2, nutrients and light on coastal plankton. III. Trophic cascade, size structure and composition

A Reul,C Sobrino,Jm Blanco,S Salles,Jm Mercado,M Segovia,Pj Neale,G Kulk,B Bautista,M Muñoz,Wh Van De Poll,E Pérez,P León,A Buma

doi:10.3354/ab00585

Abstract

We investigated the impacts of climate change-associated abiotic factors on the species composition and size structure of coastal phytoplankton communities. Surface coastal water collected off the coast of Malaga (Spain) was incubated outdoors during a 7 d microcosm experiment. The natural phytoplankton communities were exposed to high and low conditions of CO2, nutrients and light. During the first 2 d, a positive response to increased CO2 and nutrient concentration was observed in terms of abundance and chlorophyll in all size fractions (20 mu m). After 2 d, a trophic cascade effect was observed within the phytoplankton communities for all treatments. The absence of mesozooplankton led to an increase in microzooplankton abundance, which coincided with a decrease in the abundance of phytoplankton 20 mu m ESD showed a positive response to the effects of increasing CO2 and nutrient concentration. These results high light the importance of trophic interactions other than abiotic factors such as CO2 and nutrient availability in shaping the size structure of Mediterranean phytoplankton. More specifically, this work shows the importance of trophic cascade effects in scaling the plankton SAS and should be considered in both enclosure experiments and field measurements that deal with size distribution.

Highlights

Quantifying the efficiency of the ocean’s biological pump, which removes CO2 from surface waters and sequesters carbon into deep waters (Falkowski et al.2000), is still one of the priorities in oceanographic science
The initial and final composition of the phytoplankton community confirmed that diatoms dominated the phytoplankton assemblages in the >15 μm size range
In order to verify the results of the fluorospectrometer, total and diatomrelated chl a concentration were compared with independent chl a measurements

Summary

Introduction

Quantifying the efficiency of the ocean’s biological pump, which removes CO2 from surface waters and sequesters carbon into deep waters (Falkowski et al.2000), is still one of the priorities in oceanographic science. Most of the anthropogenic CO2 remains above the permanent thermocline and up to 30% remains in the upper 200 m of the water column (Sabine et al 2004). This leads to the acidification of the euphotic layer (Feely et al 2004, Sabine et al 2004, Orr et al 2005), which can affect physiological processes (Sobrino et al 2008) and the composition of the phytoplankton community (Tortell et al 2002). Apart from acidification, global warming may enhance stratification, which reduces nutrient availability in the euphotic layer by strengthening and shoaling of the thermocline (Boyd & Doney 2002, Polovina et al 2008). High nutrient inputs, both from organic and inorganic origin, are expected for most of the coastal areas where changes in land use and hydrological cycles modify the amount and nature of continental inputs into the ocean (Duce et al 2008)

Methods

Results

Discussion

Conclusion