The role of predator removal by fishing on ocean carbon dynamics

The ocean is a net sequester of carbon dioxide, predominantly through low biomass, high productivity phytoplankton photosynthesis. Selective removal of predatory fish through extractive fishing alters the community structure of the ocean, with an increased biomass of more productive, low trophic level fish and higher overall respiration rates, despite possible decreases in total fish biomass. High pressure fishing on predators may result in as much as a 19% increase in respiration from fish communities and could prove highly significant in global carbon budgets.

Quantitative relationship between solar radiation intensity and average daily value of photosynthesis light saturation for phytoplankton in the deep-water area of the Black Sea

According to data obtained during expeditions in the Black Sea (1987–1993), linear relationship between the light flux density incident on the sea surface (E0) and the starting point of photosynthesis light saturation (En opt) is revealed. For calculations, measurements of phytoplankton photosynthesis rate obtained by the radiocarbon method were used. The equation of the relationship between the values reported is presented for the first time for the Black Sea. En opt is the average daily, optimal value of photosynthesis light saturation. The parameters of photosynthesis – light curve, determined in short-period exposures under constant illumination, differ from the parameters obtained in long-term experiments under conditions of variable illumination. This is due to different effects of the intensity and dose on the phytoplankton photosynthesis rate. The values of photosynthetic parameters for a certain time are integrated into a single value which is the optimum for the entire period observed. The approximation of daily data integrated is carried out both separately for seasons and in general for the period of 1987–1993. Using statistical processing of data of average daily values of the intensity of solar radiation incident on the sea surface, slope of the photosynthesis – light curve, and maximum photosynthesis rate, the approximation is determined for the functional dependence of En opt on E0. The equation is applicable in the range of light intensity 3 to 75 mol quanta·m−2·day−1. It describes with high reliability a change of average daily value of photosynthesis light saturation in the Black Sea during different seasons of the year. The equation includes a parameter easily accessible for measurement. It can be used in analysis of physiological characteristics of phytoplankton and calculation of integrated phytoplankton productivity in euphotic layer with using both satellite and expedition data.

High-resolution underway measurements of phytoplankton photosynthesis and abundance as an innovative addition to water quality monitoring programs

Marine waters can be highly heterogeneous both on a spatial and temporal scale, yet monitoring programs currently rely primarily on low-resolution methods. This potentially leads to undersampling. This study explores the potential of two high-resolution methods for monitoring phytoplankton dynamics: fast repetition rate fluorometry for information on phytoplankton photosynthesis and productivity and automated scanning flow cytometry for information on phytoplankton abundance and community composition. These methods were tested in combination with an underway Ferrybox system during four cruises on the Dutch North Sea in April, May, June, and August 2017. The high-resolution methods were able to visualize both the spatial and temporal variability of the phytoplankton community in the Dutch North Sea. Spectral cluster analysis was applied to objectively interpret the multitude of parameters and visualize potential spatial patterns. This resulted in the identification of biogeographic regions with distinct phytoplankton communities, which varied per cruise. Our results clearly show that the sampling based on fixed stations does not give a good representation of the spatial patterns, showing the added value of underway high-resolution measurements. To fully exploit the potential of the tested high-resolution measurement setup, methodological constraints need further research. Among these constraints are accounting for the diurnal cycle in photophysiological parameters concurrent to the spatial variation, better predictions of the electron requirement for carbon fixation to estimate gross primary productivity, and the identification of more flow cytometer clusters with informative value. Nevertheless, the richness of additional information provided by high-resolution methods can improve existing low-resolution monitoring programs towards a more precise and ecosystemic ecological assessment of the phytoplankton community and productivity.

High resolution <i>in situ</i> measurements of phytoplankton photosynthesis and abundance in the Dutch North Sea

Marine waters can be highly heterogeneous both on a spatial and temporal scale, yet monitoring is currently mainly limited to low-resolution methods. This study explores the use of two high-resolution methods to study phytoplankton dynamics; Fast Repetition Rate fluorometry (FRRf) to study phytoplankton photosynthesis and scanning flowcytometry (FCM) to study phytoplankton biomass and composition. Measurements were conducted during four cruises on the Dutch North Sea in April, May, June, and August of 2017. Both FRRf and FCM data show spatial heterogeneity with monthly variation. Automated unsupervised Hidden Markov Model (uHMM) spatial clustering resulted in the identification of regions with distinct phytoplankton communities. Manual adjustments were necessary to optimize visualization of some distinct phytoplankton communities. Stepwise multiple linear regression (n = 61) revealed that photophysiology (alpha), phytoplankton biomass (total red fluorescence) and abiotic predictors (Turbidity, DIN, time of the day and temperature) determined integrated water column gross primary productivity. Apart from spatial heterogeneity, the diurnal trend is a significant predictor exposing clear trends with other photophysiological parameters. Consequently, spatial patterns are difficult as temporal and spatial patterns occur simultaneously. Nevertheless, high-resolution monitoring is a very useful supplement in addition to regular low-resolution monitoring.