engleski

Growth ratio of selected sheet-like bryozoan colonies along a latitudinal transect

Climate change is altering sea-water parameters such as near surface temperature, salinity and primary production across much of the world’s oceans. Organisms in the shallow-water areas are likely to have different energy requirements, reproductive cycle and growth rates therefore to detect the impact of climate change on ecological performance, baseline studies are of paramount importance. Yet, there are few studies on growth dynamics of shallow-water sedentary fauna such as bryozoans. Linear growth increments among erect forms of bryozoans is from 2.5 to 100 mm per year. However there is almost no information in the literature on growth rates of sheet-like colonies despite their ubiquitous and often dominant (amongst bryozoans) occurrence in the coastal zone. Here we report growth ratios of selected bryozoans from four contrasting environments, largely differing in mean annual water temperature (T) and salinity (S). The four study sites were located along the latitudinal profile from the Adriatic Sea [44° N] (T = 15.4º C, S ~ 38 PSU) in the South through the Baltic Sea [54° N] (T = 10.5º C, S ~ 7 PSU) and the northern Norway [68° N] (T = 6.7º C, S ~ 35 PSU) up to Spitsbergen [78° N] (T = 0.5º C, S ~ 34 PSU) in the North. To exclude substrate heterogeneity, bryozoans were allowed to grow on similar flat Perspex panels that were submerged in water at all sites. Panels were photographed underwater monthly, tri-monthly or annually depending on the site and the tempo of growth calculated. At the end of investigation, panels were retrieved from the water and species identification was conducted. Growth rates were analyzed backwards using the underwater digital images. We found a negative trend between growth and latitude. Within genus (or other taxonomic unit) bryozoans from lower latitudes grew faster. For example, the annual growth of species of the cyclostome genus Diposolen from the Adriatic Sea was comparable with that of Diplosolen from Spitsbergen (~ 90 cm2) after three years. Similar patterns were found among the cheilostome species of Cribrilina genus and family Calloporidea. Likewise, the Norwegian specimens grew faster than those in the Arctic waters. For example the surface area of Microporella individuals after the first year in the northern Norway was ~ 9 cm2 compared to ~ 1.5 cm2 in Spitsbergen. In the brackish environment of the Baltic Sea bryozoan Electra crustulenta was growing relatively fast for its latitudinal position and water temperature, with colonies reaching a surface area of up to 6000 cm2 in one year. Its specific life history and processes are likely related to decreased salinity were responsible for this higher growth rate. We conclude that processes related to temperature and salinity have strong impacts on growth of sheet-like bryozoan colonies. We suggest that further increases in temperature will have complex influences on growth and should be largely manifested as increases in its magnitude.

Bryozoa; growth-ration; latitudinal transect

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