Brownification reduces oxygen gross primary production and community respiration and changes the phytoplankton community composition: An in situ mesocosm experiment with high‐frequency sensor measurements in a North Atlantic bay

Abstract

In recent decades, the increase in terrestrial inputs to freshwater and coastal ecosystems, especially occurring at northern latitudes, has led to a process of water color darkening known as "brownification." To assess how brownification affects plankton community composition and functioning in northern coastal areas, an in situ mesocosm experiment using a highly colored humic substance to simulate a brownification event was performed in a North Atlantic bay (Hopavågen, Norway) in August 2019. Manual sampling for analyses of nutrient concentrations, phytoplankton pigments and zooplankton abundances was combined with high-frequency (every 15 min) monitoring of key environmental variables to investigate the response of the plankton community in terms of oxygen metabolism and community composition. In response to brownification, the oxygen gross primary production (GPP) and community respiration (R) slowed down significantly, by almost one-third. However, GPP and R both decreased to the same extent; thus, the oxygen metabolic balance was not affected. Moreover, the chlorophyll-a concentration significantly decreased under brownification, by 9% on average, and the chemotaxonomic pigment composition of the phytoplankton changed, indicating their acclimation to the reduced light availability. In addition, brownification seemed to favor appendicularians, the dominant mesozooplankton group in the mesocosms, which potentially contributed to lowering the phytoplankton biomass. In conclusion, the results of this in situ mesocosm experiment suggest that brownification could induce significant changes in phytoplankton and zooplankton community composition and significantly alter the overall oxygen metabolism of plankton communities in a northern Atlantic bay.Freshwater and marine ecosystems are naturally subject to terrestrial inputs bringing humic organic substances and inorganic nutrients during natural rainfall events (Roulet and Moore 2006;Monteith et al. 2007). However, an increase in terrestrial inputs has been observed in freshwater and coastal ecosystems in recent decades (Roulet and Moore 2006;SanClements et al. 2012) being related to changes in land cover and uses (Correll et al. 2001;Clutterbuck and

Metabolic responses of plankton to warming during different productive seasons in coastal Mediterranean waters revealed by in situ mesocosm experiments

“…The daily light integral, the quantity of photosynthetically active photons received on a 1 m 2 surface over a 1 d period 52 , was estimated using high-frequency PAR measurements at 1 m depth, with Eq. ( 1 ): …”

“…Then, hourly l rates were multiplied by 24 to obtain daily rates (d −1 ). Here, we assumed that losses occurred during the entire 24-h period, and hourly µ rates were multiplied by the duration of the increasing period, in hours, as growth only occurred during the increasing period 52 – 54 .…”

“…In this equation, k represents the piston velocity coefficient, and the concentration and saturation of dissolved oxygen respectively, and the water column mixing depth, which is the mesocosm water column length in the case of mixed mesocosms 50 , 52 . The k value was taken as k = 0.000156 m min −1 from the literature 58 .…”

Coastal darkening exacerbates eutrophication symptoms through bottom‐up and top‐down control modification

“…However, the Chl a concentration and abundance might still not fully reflect phytoplankton biomass. Combining multiple methods: pigment analyses for the community composition (Soulié et al 2022) and image analyses for the community size structure (Dunker et al 2018) could be alternative approaches in future studies to provide faster and more reliable estimation of phytoplankton biomass and composition, as manual counting requires expertise and time.…”

Browning, nutrient inputs, and fast vertical mixing from simulated extreme rainfall and wind stress alter estuarine phytoplankton productivity

“…Previous results have reported that increasing DOM concentrations impaired the photosynthesis of monospecific cultures of cyanobacterial and natural phytoplankton when combined with vertical mixing‐mediated light fluctuations (Helbling et al ., 2015). High terrestrial inputs promote changes toward communities dominated by microphytoplankton (Villafañe et al ., 2018); reductions in primary and bacterial production (Paczkowska et al ., 2020); respiration and Chl a (Soulié et al ., 2022); species richness and photosynthetic efficiency (Rasconi et al ., 2015), as well as in the trophic transfer efficiency toward consumers (Hébert et al ., 2022). By contrast, recent results obtained by Spilling et al .…”