# Sediment Characteristics in Stonewort Chara tomentosa Assemblages

## TL;DR

The study explores how the presence of Chara tomentosa affects sediment properties in the Baltic Sea, showing differences in grain size and phosphorus levels.

## Contribution

The study identifies specific sediment characteristics influenced by Chara tomentosa, offering insights for habitat restoration.

## Key findings

- Sediments with Chara tomentosa have finer grain sizes and lower dissolved phosphate concentrations.
- Variability in organic content and nutrient levels is reduced in areas with Chara tomentosa.
- Chara tomentosa habitats influence phosphorus cycling at the sediment-water interface.

## Abstract

Charophytes are green macroalgae that lay the foundation of important habitats in shallow, low‐salinity, and sheltered areas of the Baltic Sea. Charophyte habitats provide food, shelter, and reproduction sites for diverse benthic communities, fish, and waterbirds. Due to human pressures, charophytes have declined in the Baltic Sea. They are sensitive to water turbidity and eutrophication. To protect and restore charophytes and their habitats, knowledge of the species' environmental requirements is fundamental. We examined differences in sediment and porewater properties in areas with and without the large charophyte species Chara tomentosa in five shallow sheltered bays in the northern Baltic Sea. Sediment samples were analysed for total contents of phosphorus (P), nitrogen (N) and carbon (C), grain size, water content, loss‐on‐ignition, and a set of easily soluble elements (Al, B, Ba, Ca, Cd Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Sr., and Zn) from deionised water extraction. From the porewater, dissolved inorganic P (DIP) and N in nitrate and nitrite (NO2,3‐N) were analysed. The results showed significantly lower porewater DIP concentrations and finer grain sizes in the sediments from areas with
C. tomentosa
compared to those without it. The variability of the organic content and the total contents of P, N, and C was lower in the sediments from the seafloor areas occupied by
C. tomentosa
than in those without it. The contents of easily soluble elements were similar in all sediments. We conclude that the sediment grain size and organic content should be considered in charophyte restoration. The results show that
C. tomentosa
habitats influence phosphorus cycling in the sediment–water interface. How microbial communities associated with charophytes influence phosphorus cycling needs further attention to estimate the link between charophyte habitats and eutrophication.

The study investigated sediment characteristics associated with Chara tomentosa. The results show that sediment grain size and the concentration of the dissolved phosphate in the porewater differ in the seafloor areas characterised by
C. tomentosa
from adjacent areas at similar depth, but without charophytes. The invaluable charophyte habitats are threatened in the shallow coastal bays and lagoons of the Baltic Sea. A thorough understanding of the species' environmental requirements, including the sediments, is fundamental for successful restoration.