Indicator species means a species which tells something important about its environment or the environmental conditions where it thrives. Bladder wrack Fucus vesiculosus deteriorates in turbid water and the plants move towards the surface to catch more light, blue mussels need at least 4-5 per mill salinity to survive, and Vaucheria sp filamentous algae grow well in eutrophicated sheltered areas.
Water framework directive WFD aims to good ecological water quality and one of the indicators in Finland is the bladder wrack. It’s depth zone is monitored, and it seems to correlate quite well with water turbidity (or the lack of it), which correlates quite well with the amount of planktonic algae in the water column, which correlates quite well with the nutrient load in the water, which correlates quite well with the water quality. But there’s always a But. Bladder wrack depth preference doesn’t really correlate that well with anything in the Archipelago Sea. The reasons are not well understood, but in this area, the species is not as strong an indicator as elsewhere. And another But: Bladder wrack doesn’t exist in the Bothnian Bay and in the SEAmBOTH area at all so we can’t use that species to indicate anything, besides the lack of it indicating too low salinity for its growth.
It’s been a long and rocky road looking for environmental indicator species in the Bothnian Bay. One Master’s Thesis (Takalo, 2005) came to the conclusion that the vascular plants are not very useful indicators (of anything) in the Northern Bothnian Bay because of the special environmental and geographical circumstances (constant and fast land-uplift, abundance of river water, long ice cover and the low number of vascular plant species due to the aforementioned facts). Takalo does mention that epiphytic algae or Diatoms might be used as indicators of eutrophication. Another Master’s Thesis (Lantto, 2016) looked into the possibility of using filamentous algae to indicate eutrophication in the Northern Baltic Sea. Lantto did find a correlation between the two, but warns us not to interpret the correlation too lightly – it could be caused by other environmental factors (for example temperature) and not eutrophication alone. Diatoms form a large biomass in the Northern Bothnian Bay, but studying them requires special identification skills and special microscopes and hasn’t been studied yet.
One thing we can say for sure: there are some species even in the SEAmBOTH area that indicate something. Nitella wahlenbergiana most definitely indicates a river estuary, while Crassula aquatica indicates a mud flat or some other very shallow area where water level fluctuates, and Chara aspera might indicate a sand bank. What is still problematic with these indicator relationships is that for example Chara aspera might also grow in other environments as well, and for example Potamogeton friesii seems to prefer two very opposite habitats – muddy and sheltered black water ponds with soft bottom on the other hand, and clear water pebbly or sandy bottom, quite exposed areas in the outer archipelago on the other.
Trying to make indicator species out of any species is like trying to categorize every patch of nature into nature types. All of them just don’t fit in to the special little niches we try to push them into. Regardless of the difficulty of categorizing them, the species will keep growing and leave us wondering what they tell us about their surrounding nature, if anything.
Written by Essi Keskinen