Assessing long term change of Fucus spp. communities in the northern Baltic Sea using monitoring data and spatial modeling

Abstract

Changes in nutrient and sediment concentrations alter the light availability on the seafloor, and consequently the aquatic ecosystems. Decreasing light availability limits plant growth and causes changes in species composition. Local changes in marine biodiversity caused by human activities have been documented around the Baltic Sea, but large-scale changes in species’ spatial distributions remain unreported due to a lack of data and methodological solutions. The available monitoring data of the underwater nature and the new applications for the modeling of species distribution has enabled the evaluation of past environmental conditions. We have utilized two separate analysis processes with separate datasets to assess the reduction of suitable seafloor areas for Fucus spp. in the Northern Baltic Sea. The change in depth distribution of Fucus spp. was first analyzed using monitoring data for the lower growth limit of Fucus vesiculosus from 70 monitoring stations along the Finnish coast; the measurements were taken between 1936 and 2017. The second analysis was performed by modeling the current and past favorable seafloor areas for a Fucus spp. dominated biotope. Using Secchi depth data from the early 20th century to the present day, we created a spatial variable, which represents light availability on the seafloor for over a century. The analysis results of the measurements and modeling reveal a dramatic decrease of suitable seafloor for Fucus spp.. Site by site evaluation of F. vesiculosus lower growth limit shows a strong reduction when comparing older measurements from literature sources to more recent monitoring. The change in the lower growth limit was evaluated using the methodologies of the International Union for Conservation of Nature (IUCN) and Water Framework Directive (WFD) with results indicating severe degradation and a negative development towards a collapsed state. The observed long term trend could result in the habitat reaching a collapsed state within 10–15 years. Modeling results derived with the centennial Secchi data indicate on average that the decrease in light availability has reduced favorable sea floor areas for the Fucus spp. dominated biotope by 45%. Even though the Fucus spp. dominated biotope is still relatively common along the Finnish coastal areas, the drastic scale of change makes it a threatened species group.