How Well Does Chlorophyll Explain the Seasonal Variation in Phytoplankton Activity?

Maren Moltke Lyngsgaard,Stiig Markager,Eva Friis Møller,Katherine Richardson,Hans Henrik Jakobsen

doi:10.1007/s12237-017-0215-4

Maren Moltke Lyngsgaard, Stiig Markager + Show 3 more

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https://doi.org/10.1007/s12237-017-0215-4

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Abstract

The seasonal variation in phytoplankton activity is determined by analysing 1385 primary production (PP) profiles, chlorophyll a (Chl) concentration profiles and phytoplankton carbon biomass concentrations (C) from the period 1998–2012. The data was collected at six different stations in the Baltic Sea transition zone (BSTZ) which is a location with strong seasonal production patterns with light as the key parameter controlling this productivity. We show that the use of Chl as a proxy for phytoplankton activity strongly overestimates the contribution from the spring production to annual pelagic carbon flow. Spring (February and March) Chl comprised 16–30% of the total annual Chl produced, whereas spring C was much lower (8–23%) compared to the annual C. Spring PP accounted for 10–18% of the total annual PP, while the July–August production contributed 26–33%, i.e. within the time frame when zooplankton biomass and grazing pressure are highest. That is, Chl failed in this study to reflect the importance of the high summer PP. A better proxy for biomass may be C, which correlated well with the seasonal pattern of PP (Pearson correlation, p < 0.05). Thus, this study suggests to account for the strong seasonal pattern in C/Chl ratios when considering carbon flow in coastal systems. Seasonal data for PP were fitted to a simple sinusoidal wave model describing the seasonal distribution of PP in the BSTZ and were proposed to present a better parameterizaton of PP in shallow stratified temperate regions than more commonly applied proxies.

Highlights

The phytoplankton spring bloom is the most conspicuous manifestation of pelagic phytoplankton activity in temperate and polar waters and has received much attention. Platt and Sathyendranath (2008) referred to the Bspring bloom of phytoplankton^ as Bthe most important event in the trophic calendar of the pelagic systems^
The data was collected at six different stations in the Baltic Sea transition zone (BSTZ) which is a location with strong seasonal production patterns with light as the key parameter controlling this productivity
We show that the use of chlorophyll a (Chl) as a proxy for phytoplankton activity strongly overestimates the contribution from the spring production to annual pelagic carbon flow

Summary

Introduction

The phytoplankton spring bloom is the most conspicuous manifestation of pelagic phytoplankton activity in temperate and polar waters and has received much attention. Platt and Sathyendranath (2008) referred to the Bspring bloom of phytoplankton^ as Bthe most important event in the trophic calendar of the pelagic systems^. The high biomass of phytoplankton grazers during summer suggests that the pelagic primary production at this time of the year could be top-down controlled, i.e. that the primary production will not result in a build-up of phytoplankton biomass. At any given point in time, biomass will be a function of the previous concentration plus gains (growth rate and advection) and loss rates (mortality generated through cell decay and grazing processes, advection and sedimentation). Of these processes, only growth rate can be expected to be directly correlated

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