PhD defence about marine copepods' response to low salinity

The salinity in the brackish Baltic Sea is generally considered to be a barrier for both marine and freshwater species, and the Baltic Sea is therefore poor on species with low ecological resilience. Despite the brackish conditions, copepod species composition consists of freshwater, brackish and marine species, and the marine copepod species tend to dominate in terms of abundance and energetic value for fish larvae and smaller fish. Thus, the tolerance of marine species is of significant interest in predicting future changes in the Baltic ecosystem.

Predictions for climate change in the Baltic Sea includes increasing temperature, potential increasing precipitation and increasing freshwater inflow due to run-off and snow/ice melt, resulting in a decreasing salinity. Changes in salinity are expected to impact the abundance and performance of the marine species in the Baltic Sea, yet very little is known about the tolerance of the Baltic populations of marine copepod species.

The overall objective of Anette Maria Christensen's PhD project was to investigate what kinds of effects do the projected changes in the salinity of the Baltic Sea cause to one of the important marine copepods (Temora longicornis), known to be the main food sources of many commercially important fish species, and whether the marine copepods are able to acclimatize or adapt to a reduction in salinity.To answer this, she investigated the effects of a reduced salinity on vital rates and physiological responses of copepods originating from different areas of the Baltic Sea (with different native salinities). She also included methods of quantitative genetics to elucidate whether the wide distribution of marine copepods could better be explained by a broad species-specific physiological plasticity or by local adaptations.

The thesis addresses the following overarching questions:

1. What is the tolerance limit for Temora longicornis originating from marine conditions to abrupt salinity changes and would the potential underlying physiological processes allow for a survival under Baltic salinity conditions?
2. Do different Temora populations have different tolerance limits and what physiological processes may govern their potential population-specific tolerance?
3. Does the rate of salinity change and long-term acclimation expand the tolerance via developmental plasticity or are the Baltic populations of T. longicornis an example of a local adaption?
4. How representative is T. longicornis of a marine copepod, and to what extent does the marine copepod Acartia longiremis exhibit similar population-specific responses?

The presented thesis consists of five chapters that aid in the overall understanding of salinity tolerance in marine copepods. 

1. The first chapter serves as a general introduction outlining the motivations and plot for the study.
2. The second chapter addresses the hydrography and biodiversity of the Baltic Sea to frame the conditions that Baltic marine copepod species cope with - including the vulnerability of the Baltic ecosystem. 
3. The third chapter addresses different mechanisms for salinity tolerance. 
4. The fourth chapter addresses the rationale behind quantitative genetics and ‘common garden’ experiments as a method and its applications. 
5. The fifth chapter addresses the implications of the results in context to the projected changes in climate and hydrography.

The following chapters are manuscripts:

1. Manuscript 1 reports the investigations of the effects of an abrupt salinity change on different genders and life-stages of Temora longicornis originating from polyhaline conditions (Kattegat). 
2. Manuscript 2 describes the examinations of the effects of gradual acclimation to salinity conditions found in the (central) Baltic Sea, and the investigated T. longicornis populations originated from poly- and mesohaline areas, i.e. from Kattegat, Bornholm- and Gotland Basin, respectively. 
3. Manuscript 3 elucidates whether longer-term (multi-generational) acclimation could expand tolerance to match the salinity conditions found in the Baltic Sea. 
4. Manuscript 4 reports the investigations of a similar salinity shock and short-term acclimation study with Acartia longiremis to determine whether the tolerance and physiological response found in Baltic Temora could be a general representative response of marine copepod species in the Baltic Sea.