Shedding light on plankton trends: seasonal pelagic monitoring in Kongsfjorden and Isfjorden

Research notes

Two fjords in western Spitsbergen are hotspots for both marine biomass production and Arctic research. These fjords contain a mix of Atlantic and Arctic water masses and glacial meltwater from land, and both host top research facilities, making them ideal places to study what drives marine production.

By: Allison Bailey, Philipp Assmy, Ane Cecilie Kvernvik, Anette Wold, Haakon Hop, Harald Steen // Norwegian Polar Institute. Clara J.M. Hoppe // Alfred Wegener Institute. Janne E. Søreide, Anna Vader, Cheshtaa Chitkara // The University Centre in Svalbard.

Kongsfjorden is adjacent to the world-class, permanent research community Ny-Ålesund. Isfjorden, the largest fjord in Spitsbergen, borders on Svalbard’s largest permanent settlements and is readily accessible from the University Centre in Svalbard (UNIS), where Arctic research has expanded considerably in the past decades. Both fjords integrate oceanographic inputs from Atlantic and Arctic waters, and meltwater from adjacent glaciers and permafrost, and are thus prime locations to study key drivers of marine production that are relevant across the Arctic.

A 25-year-long zooplankton monitoring programme in Kongsfjorden has provided one of the longest datasets on an Arctic marine ecosystem. In Isfjorden, data on plankton communities collected over time and space have been compiled into time series (IMOS; Isfjorden Marine Observatory Svalbard), some of which reach back in time 20 years, covering spring, summer, autumn and winter.

However, a compilation of the extensive, interdisciplinary research that has been conducted on the marine environment of Kongsfjorden over the last decades made it clear that critical information about the base of the food web is missing. The primary and secondary production in the Arctic occurs as distinct pulses during the short growing season, with large interannual variability in magnitude and timing. Data with high temporal resolution is therefore required to differentiate natural seasonal and interannual variability from changes due to climate change.

In both Kongsfjorden and Isfjorden, monitoring efforts are increasingly focused on capturing the seasonal variability of the entire suite of hydrography, chemistry, phytoplankton and zooplankton. In 2019 and 2020, researchers from the Norwegian Polar Institute and the Alfred Wegener Institute conducted five-month long field campaigns investigating seasonal trends in phyto- and zooplankton at the base of the food web, and the environmental factors that influence their growth.

The Isfjorden–Adventfjorden time series (IsA) station was established in late 2011 and hydrography, water and net samples have been sampled every month and periodically every week since then. Besides classical morphological analyses, molecular tools (DNA metabarcoding) are used for increased taxonomic resolution of microbes and phytoplankton, aiming to understand species and ecosystem responses to a changing Arctic environment.

The phytoplankton spring bloom is of utmost importance as energy for higher trophic levels. However, the complex interacting factors that influence spring bloom timing, magnitude, composition, and palatability for zooplankton grazers limits our ability to predict energy transfer to higher trophic levels. The characteristics of the water mass and sea ice cover are known to play significant roles in the timing and magnitude of the spring bloom, but large interannual variability and the scarcity of seasonal data make it difficult to detect any long-term trends.

Shifts in the timing of the bloom can cause match or mismatch with regard to zooplankton reproduction and development of their early life stages. In addition to timing, the species composition of the spring bloom can differ, with associated effects on nutrient concentrations in the fjord, quality of the phytoplankton as food for herbivorous zooplankton, and the strength of the biological carbon pump.

In Kongsfjorden, high resolution phytoplankton taxonomy data from the past two summers have revealed the rise and fall of different phytoplankton species during the seasons and shown that the dominant species of the spring bloom can differ drastically from one year to the next. In 2019, the flagellate Phaeocystis pouchetii dominated the spring bloom in Kongsfjorden, while the spring bloom the following year, 2020, was dominated by diatoms of the genera Chaetoceros, Bacteriosira and Thalassiosira. Associated abiotic differences are currently being analysed to understand what may drive these differences, and what effect, if any, they have on the wider ecosystem.

For example, data from the Norwegian Polar Institute show that the water was colder in late spring of 2020 than in 2019. In the warm spring of 2019, the early feeding stages of the dominant Calanus zooplankton coincided with the spring bloom, providing good conditions for growth.

We await the data for 2020 to see how the timing of the spring bloom and the main copepod grazer aligned in the colder spring of 2020. During summer, the pelagic ecosystem in Kongsfjorden is subject to extensive meltwater runoff from glaciers, which has a marked impact on fjord circulation, the marine light climate, and nutrient dynamics. Late July 2020 saw record-high air temperatures and record glacier melt in Kongsfjorden.

Based on the Isfjorden–Adventfjorden time series, seasonal and interannual variations in the community composition and biodiversity of unicellular organisms during 2011-2019 are currently being analysed. Our data show a highly seasonal pattern with large interannual variations in community composition. The years 2012-2014 varied greatly in “fjord climate”: 2013 was a cold, 2014 a warm, and 2012 an intermediate year. The timing and magnitude of the spring bloom also varied greatly between these years, as did the species composition of the community. Micromonas polaris, a small green alga adapted to cold water and shade, is an important baseline phototroph in the Arctic.

By using DNA metabarcoding we could determine that this arctic species was replaced by a boreal species of Micromonas in the warm year 2014.

Documenting seasonal changes in the Arctic is key to detecting long-term trends and to obtaining the richness of data needed to reveal diverse responses of the marine ecosystem to a changing environment. Seasonal monitoring is logistically demanding, but it allows us both to make more precise estimates of yearly biomass production for comparison on interannual timescales, and also to track the timing of key biological events such as the phytoplankton spring bloom, which are expected to be altered with climate change.