Comparing static and dynamic incubations in primary production measurements under different euphotic and mixing depths

Ulf Båmstedt

doi:10.1007/s10750-018-3762-1

Abstract

Since phytoplankton production is usually estimated from static incubations (fixed depths or light levels), a mesocosm study was performed to evaluate the significance of mixing depth, mixing intensity and load of humus of natural phytoplankton assemblages. Vertically rotated (dynamic) incubations usually gave higher results than static incubations in humus-rich water. Mixing intensity was of significant importance in one of 2 years tested, but strong interaction effects with humus complicated the explanation. Differences in primary production between dynamic incubations did not fully reflect the received PAR dose, and increased humus and increased mixing depth increased the photo-assimilation efficiency. Different single-depth incubations did not provide a shortcut method to measure water-column primary production with high accuracy. Results diverged from theoretical estimates based on recent combined photo-biological and physical environmental models. The large variability in responses to mixing is supposed to reflect species-specific adaptations and pre-history regarding quantity (photons) and quality (spectral distribution) of the optical environment in an assemblage of different species. The proportional abundance of each species with its specific characters will therefore strongly influence bulk primary production. Due to such variable responses, clear guidelines for a “best practice” in primary production measurements cannot be given, based on the present results.

Highlights

Measuring primary production in the field includes trapping a bulk sample of the phytoplankton community in transparent bottles and incubating for a given time, using a series of fixed depths or irradiance levels or by using on-deck incubators that provide a range of different irradiance levels, comparable to a vertical profile of the in situ irradiance levels
The euphotic zone, here defined as the depth of 1% surface irradiance, differed between the 2 years in the experiments with no humus addition, where the whole water column was above the euphotic zone in 2013, but 1/5 of the water column received less than 1% of the surface irradiance in 2014 (Table 2)
The euphotic zone was reduced to approximately the same depth in both years, with around 40% of the water column below the euphotic zone with medium humus and around 60% below the euphotic zone with high humus (Table 2)

Summary

Introduction

Measuring primary production in the field includes trapping a bulk sample of the phytoplankton community in transparent bottles and incubating for a given time, using a series of fixed depths or irradiance levels or by using on-deck incubators that provide a range of different irradiance levels, comparable to a vertical profile of the in situ irradiance levels. Phytoplankton in the upper mixed layer are probably experiencing variable light conditions that are poorly mirrored by phytoplankton held in bottles at fixed depth or in incubators. Since phytoplankton photosynthesis is mediated by physiological adaptations to different irradiance levels (Falkowski, 1983), their responses to vertical mixing will depend primarily on the intensity of mixing. Since the spectral distribution changes with depth and depends on the content of e.g. humic substances, a deck incubator with neutral filters for different irradiance levels will not properly reproduce the light in a natural water column (Walsh & Legendre, 1983; Bertoni & Balseiro, 2005). Due to continuous turbulent mixing, a water sample taken from a single depth will contain algal cells with different light histories and different photo-acclimation states (Ross et al, 2011a)

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