Changes in sinking of plankton--like particle: comparison between observations and numerical model

TL;DR

This study compares observed seasonal variations in diatom sinking and distribution with numerical models, revealing that buoyancy changes influence sinking speeds differently in winter and summer.

Contribution

It introduces a numerical model that links seasonal environmental conditions to diatom sinking behavior, validated against long-term observational data.

Findings

Sinking speeds decrease in winter conditions according to the model.

Seasonal wind influences diatom surface concentrations differently in summer and winter.

Numerical simulations support the hypothesis that buoyancy changes affect sinking rates.

Abstract

Sinking and resuspension are the most important processes determining the distribution of bentho--pelagic diatoms in the water column and influencing their variability. From the bentho--pelagic time series 1991-2008 we analyzed weekly concentrations of the diatom Paralia sulcata in the German Bight. The analysis was carried out for lower wind ranges (<10 m/s) at two different seasons characterized by distinct sea surface conditions. Our results showed that the relationship between wind and diatom concentrations was much influenced by seasonal conditions: the decrease of abundances at higher winds was more pronounced in summer. Additional sampling procedure of species at two different water depths (bottom and surface layers) in the years 2007-2009 showed that in summer the surface abundance of species decreased substantially with increasing wind. In contrast, in winter the decrease of the surface concentration of species was less pronounced. To explain observed seasonal variations numerical simulations of plankton--like particles were performed using a two--dimensional turbulence model. Numerical simulations were carried using parameters corresponding to averaged winter and summer temperature and salinity at the study site. We showed that the reduction of cell counts was possibly linked to the changes in sinking of diatoms due to the altered buoyancy of algal cells. Our numerical results identified reduction of sinking speed of particles for the parameters corresponding to winter conditions.