Doctoral defence: Heli Einberg “Non-linear and non-stationary relationships in the pelagic ecosystem of the Gulf of Riga (Baltic Sea)”

On 4 February at 12:15 Heli Einberg will defend her doctoral thesis “Non-linear and non-stationary relationships in the pelagic ecosystem of the Gulf of Riga (Baltic Sea)” for obtaining the degree of Doctor of Philosophy (in Hydrobiology and Fishery).

Supervisors:
Doctor Riina Klais-Peets, Ministry of Environment
Professor Henn Ojaveer, University of Tartu

Opponent:
Senior Research Fellow Benjamin Planque, Institute of Marine Research in Norway (Norway)

Summary
This thesis focuses on non-linear and non-stationary relationships in the Gulf of Riga (Baltic Sea) pelagic ecosystem. The relationships in the food webs or species-environment relationships may be linear or non-linear, while the type, strength and direction can also change over time (i.e. be non-stationary). Although the comprehension that long-term relationships in marine ecosystems are rarely stable and linear is widespread, still quite often the statistical methods that are used to identify these links, i.e. cannot detect non-linearity and non-stationarity. This is also true for the Baltic Sea, where the application of more advanced statistical methods and reporting of non-linear and non-stationary results has grown only over the last decade. The long-term environmental and biological monitoring data from the Gulf of Riga used in this thesis span almost 60 years (1958-2018), and include some previously unavailable data. Over this period, the Gulf of Riga ecosystem has undergone gradual as well as abrupt changes (so-called regime shifts). Publications of this thesis include three case studies: i) long term abundance dynamics of the arctic copepod Limnocalanus macrurus identification of the likely drivers of change, ii) changes in the abundance dynamics and alteration of the prey-environment relationships of the primary prey taxa of the non-indigenous predator cladoceran Cercopagis pengoi after its invasion, and iii) factors affecting the recruitment dynamics of the local spring spawning herring (Clupea harengus membras) population. In the statistical analyses, a suite of the analytical methods was used including the generalized additive models, and two different “moving window” methods. Many of the results in this thesis provided new and previously unknown insights to the functioning of the Gulf of Riga ecosystem.