The view of a data scientist - BONUS BIO-C3 & BONUS INSPIRE
Why is Baltic Sea more productive after mild winters?
Small copepods in Baltic Sea
In Baltic Sea, zooplankton production tends to be higher in springs and early summers that follow milder winters. Zooplankton are the tiny (usually shorter than 1 cm) animals in the surface layers of the seas. They channel the primary production to larger animals. This link between warmer winters and zooplankton production is known for a while, but the mechanism behind it is still not clear.
We're now trying to shed some light into this.
Pärnu Bay is a small and shallow bay in the north-east corner of Gulf of Riga. It's the summer destination for most Estonians (and also Finns), for its long sandy beaches and warm weather.
But it's also a place, where Estonian Marine Institute has longest been sampling the zooplankton - for more than 60 years now!
We're looking this time on small copepods. Small copepods are quite an intriguing zooplankton group. They are like tiny shrimps (usually less than 1.5 mm long), and main food for fish larvae in many ecosystems. In Pärnu Bay, most common representatives of small copepods are Eurytemora affinis, and two or three members of the genus Acartia (A. bifilosa, A. tonsa).
What makes the small copepods especially interesting, is that they’ve been overlooked for long time in many parts of the seas, globally! Zooplankton was (and still is) in many places sampled with a net that has too large mesh size to capture small copepods. When finer nets are used, small copepods tend to be quite, if not most, important -- most abundant, and making most of the biomass too.
Why is climate change research tricky?
Just because some things appear to be correlated in nature over long time, does not yet prove that they are causally linked. This is opposite to the lab experiments, where all effects are controlled, and the correlations are usually causal.
Therefore, for me, most interesting and challenging part of this study is to try to figure out what's the deal. Showing once again that warmer winters are followed by higher production of copepods, and that the best predictor is winter air temperature, won't bring anyone a Nobel prize, this is already known.
Instead, I am taking a closer look on this link itself, on the behaviour of the model. How consistent is this correlation, when looking shorter periods during the last six decades? Does it depend on which years are included in the data? Has this link been stronger or weaker in the past? Is it stronger or weaker in the periods of generally milder or colder winters?
Zooplankton observations in the Pärnu Bay go back as far as 1957, and we've seen both cold and warm winter since then. This gives me plenty of variability to play with.
Another interesting question, more pertinent to policy makers and managers, is whether the links that we find using data from the past, can forecast zooplankton in the future. First and foremost goal of climate change research is to predict the future! I'm still working on my analyses, and I will update this post ASAP!
In case you will be participating ICES/PICES conference of zooplankton in Bergen, I will talk about this study there (and likely the very last results from the evening before the conference).
Winter of 2015/16 was again very mild, based on James Hurrell NAO index, so we look forward lots of small copepods this spring in Pärnu. As do the larvae of spring herring!