# Environmental Drivers and Long-Term Dynamics of Copepod Communities in the Black Sea: Contrasts Between Warm and Cold Periods

## TL;DR

This study examines how copepod communities in the Black Sea changed over 60 years, showing declines linked to environmental factors like temperature and nutrients.

## Contribution

The study reveals distinct seasonal and long-term shifts in copepod communities driven by climate and human-induced pressures in the Black Sea.

## Key findings

- Copepod density and biomass declined significantly from the 1960s to 2000, especially during colder months.
- Phosphate levels positively influenced copepod abundance in warm seasons, while temperature and salinity played key roles in cold seasons.
- Species composition shifted over time, with fewer constant species and more rare or accidental taxa in later decades.

## Abstract

Copepods play a crucial role in marine food webs, as they feed on microalgae and are food for many fish species. The paper investigates the dynamics and the key drivers of changes in the copepod community along the Romanian coast of the Black Sea over 60 years (1956–2015). We assess seasonal and long-term changes in density, biomass, and species composition, and explore their relationships with environmental variables, including temperature, salinity, nutrients, and oxygen. The results marked decline in copepod density, biomass, and species richness starting in the early 1980s, especially during colder months Seasonal variation in species composition is evident, with distinct communities characterizing warm and cold months. These seasonal patterns are influenced not only by environmental conditions but also by shifts in the copepod community itself, as changes in the relative abundance of dominant and opportunistic species modulate overall community structure across seasons. Among environmental drivers, increased nutrient concentrations, especially phosphates and ammonium, emerged as major factors influencing copepod community structure. These findings enhance our understanding of long-term dynamics in marine zooplankton and highlight the sensitivity of copepod communities to both climatic and human-induced pressures. Moreover, they underscore the value of copepods as effective bioindicators of marine ecosystem health and provide insights to inform conservation strategies and the management of ecosystems in the Black Sea region.

Copepods are key components of marine food webs, linking primary producers such as microalgae to higher trophic levels, including many fish species. This study investigates long-term changes in the composition, density, and biomass of copepod communities along the Romanian coast of the Black Sea over six decades (1956–2015), based on historical records and recent monitoring from 18 sampling stations. Mean copepod density declined markedly over the study period, particularly during the cold season, decreasing from values exceeding 1000 ind/m3 in the 1960s to <300 ind/m3 after 2000, while biomass showed weaker but comparable long-term fluctuations. Seasonal variability was pronounced, with significantly higher densities and biomass during the warm season. Generalised Additive Models (GAMs) explained up to 40–55% of the variance in copepod density and biomass, depending on the season. During the warm season, phosphate exerted a positive effect on copepod abundance, consistent with bottom-up control via phytoplankton productivity, whereas during the cold season, temperature showed a positive effect and salinity a negative effect, indicating stronger physical control of copepod persistence. Species composition shifted over time, with a reduction in constant species and an increase in rare or accidental taxa in later decades. These results indicate that climate variability and anthropogenic pressures have reshaped copepod communities, with potential consequences for food-web efficiency and ecosystem resilience in the Black Sea.