Summer variability in bio-optical properties and phytoplankton pigment signatures in two adjacent high Arctic fjords, Svalbard

Fjords in Svalbard are sensitive to the enhanced climate warming-induced glacial meltwater discharge (fresher, nutrient-rich, turbid) that influences its hydrography and biological processes. This study attempts to explain the impacts of underwater light environment and nutrient limitation on the phytoplankton biomass, composition, and light absorption in the sunlit zone of the Kongsfjorden (KG)-Krossfjorden (KR) twin fjord ecosystem during summer when the meltwater discharge is at maximum. Observations conducted in two phases in each fjord revealed pronounced spatial hydrographic variations between the phases and among the fjords. Intrusion of warm Atlantic water into both fjords and occurrence of subsurface chlorophyll maxima were observed. Meltwater-induced higher concentrations of optically active constituents in KG resulted in a comparatively shallow euphotic zone than KR. Nitrate and silicate limitation (N/P < 5, N/Si > 1, Si/P < 3) was evident in both fjords. Higher phytoplankton light absorption coefficient (a(ph), m(-1)) and chlorophyll-a in KR implied its higher productivity potential. However, the light absorption efficiency of microplankton (at surface) was affected by pigment package effect. Phytoplankton pigments analyses revealed an inter-fjord difference in surface phytoplankton composition predominated by microphytoplankton (49-100%) followed by pico- (0-38%) and nanoplankton (0-18%). The average diversity index for phytoplankton group (H') was higher in KR (0.71) than KG (0.55), which was possibly controlled by microzooplankton grazing. This study reveals that environmental settings in both fjords were quite different, which drives their productivity potential and species diversity. Thus, increased warming climate can have different levels of impacts on fjord ecosystems despite their close geographical proximity.