Phytoplankton dynamics in the Barents Sea estimated from chlorophyll budget models

Abstract

Pigment budgets use chlorophyll a and phaeopigment standing stock in combination with their photo-oxidation and sedimentation rates in the euphotic zone to estimate phytoplankton growth and grazing by micro- and macrozooplankton. Using this approach, average phytoplankton growth in the euphotic zone of the Barents Sea was estimated at 0.17 and 0.14 d?1 during spring of 1987 and 0.018 and 0.036 d?1 during late- and postbloom conditions in summer of 1988. Spring growth was 65% lower than the estimates from radiocarbon incorporation, supporting a 33% pigment loss during grazing. Macrozooplankton grazing and cell sinking were the main loss terms for phytoplankton during spring while microzooplankton grazing was dominant in summer. In contrast to tropical and temperate waters, Arctic waters are characterized by a high phaeopigment: chlorophyll a ratio in the seston. Photo oxidation rates of phaeopigments at in situ temperatures (0 ± 1°C) are lower than in temperate waters and vary by a factor of 2 for individual forms (0.009 to 0.018 m?2mol?1). The phaeopigment fraction in both the suspended and sedimenting material was composed of seven main compounds that were isolated using high-performance liquid chromatography and characterised by spectral analysis. The most abundant phaeopigment in the sediment traps, a phaeo-phorbide-like molecule of intermediate polarity (phaeophorbide a3), peaked in abundance in the water column below the 1% isolume for PAR (60-80 m) and showed the highest rate of photo oxidation. This phaeopigment was least abundant in the seston when phytoplankton was dominated by prymnesiophytcs but increased its abundance in plankton dominated by diatoms. This distribution suggests that larger grazers feeding on diatoms are the main producers of this phaeopigment.