# Distribution of Summer Zooplankton in the Waters off the Kuril Islands (Northwest Pacific) in Relationship with Environmental Conditions

## TL;DR

This study explores how environmental factors like temperature and salinity influence zooplankton distribution around the Kuril Islands in the Northwest Pacific.

## Contribution

The study identifies specific environmental variables driving zooplankton abundance and diversity across different water masses in a climatically diverse region.

## Key findings

- Zooplankton abundance and biomass are higher in Pacific waters compared to the Sea of Okhotsk.
- Water temperature and salinity below the thermocline are key drivers of zooplankton distribution.
- Large copepods correlate with temperature, salinity, oxygen, and chlorophyll a, while small copepods thrive in warmer, saltier, and less productive waters.

## Abstract

In this study, we investigated the key environmental factors driving longitudinal changes in zooplankton assemblages around the Kuril Islands, Northwest Pacific. The spatial and vertical variations in the composition of zooplankton assemblages were attributed to parameters of water masses. The water temperature and salinity below the thermocline were the primary drivers of zooplankton abundance. The same environmental factor might have different effects on the abundance of boreal species. These results enhance our understanding of how marine environment conditions cause changes in planktonic communities and highlight the ecological importance of zooplankton as a natural indicator of environmental quality.

This study covers a large area around the Kuril Islands, one of the longest island arcs in the world, crossing several climatic zones, which allows for observations of longitudinal shifts in planktonic species’ ranges following shifts in the boundaries between the climatic zones. We analyzed spatial and vertical changes in the zooplankton community structure and the associated environmental factors from Yuri Island to Onekotan Island both in Pacific and Sea of Okhotsk waters, which are influenced by cold and warm water masses. Species diversity in the Pacific waters was higher than in the Sea of Okhotsk waters, with a peak of diversity recorded from warm waters off the southern Kuril Islands associated with the Soya Current and the Kuroshio Extension. Zooplankton abundance and biomass above the thermocline were higher in the Pacific waters compared to the Sea of Okhotsk area, showing a tendency to increase with higher latitude and lower water temperatures and generally to decline with depth. The water temperature and salinity below the thermocline were the most important explanatory environmental variables responsible for zooplankton abundance variations. The distribution patterns of the large-sized copepods were strongly correlated with temperature and salinity, as well as with dissolved oxygen and chlorophyll a levels. In contrast, small-sized copepods were adapted to or thrived in the areas with elevated temperature and salinity values and a reduced chlorophyll a concentration.