Stochastic and Deterministic Processes Regulate Phytoplankton Assemblages in a Temperate Coastal Ecosystem.

Area of Science:

• Marine Ecology
• Community Ecology
• Phytoplankton Ecology

Background:

• Understanding phytoplankton community assembly is vital for predicting ecological organization and succession.
• Deterministic and stochastic processes interact to shape ecological communities.

Purpose of the Study:

• To investigate the seasonal patterns of deterministic and stochastic processes in phytoplankton community assembly.
• To determine the relative contributions of environmental filtering and ecological drift over a 5-year period.

Main Methods:

• A 5-year survey in a temperate coastal ecosystem (eastern English Channel).
• Quantification of phytoplankton abundance and biomass using microscopy and flow cytometry.
• Metabarcoding and null model analysis to evaluate diversity and ecological processes.

Main Results:

• Phytoplankton alpha diversity was primarily influenced by environmental filtering.
• Temporal community turnover showed a consistent interannual seasonal pattern.
• Stochastic processes (ecological drift) dominated phytoplankton assembly for most of the year (April-December, 55%), while deterministic processes (homogeneous selection) were stronger in winter/early spring (January-March, 38%).
• Recurrent, transient monospecific phytoplankton peaks were observed, particularly in late spring and summer.

Conclusions:

• Phytoplankton community dynamics exhibit predictable seasonal patterns driven by a balance of deterministic and stochastic processes.
• The prevalence of stochastic processes suggests greater unpredictability in future phytoplankton seasonal dynamics.
• This study provides a long-term, pluriannual perspective on the interplay of ecological processes governing phytoplankton succession.