All species compete for resources, including nutrients and, in the case of microalgae, light. The outcome of this competition can lead to coexistence or exclusion of competitors. Understanding whether harmful microalgae can invade communities that currently consist only of non-harmful microalgae therefore requires not only an understanding of their tolerances to temperature or salinity, but also an understanding of the conditions under which coexistence is possible or under which competition leads to the exclusion of a competitor. To gain this understanding, we aim to determine the competition between harmful and non-harmful microalgae under different scenarios, including predicted temperature increases, predicted salinity changes and in the presence of the main consumers of microalgae, copepods. This information, together with the other work packages (WPs), will help to develop a mechanistic understanding of whether and under what conditions harmful species can invade plankton communities in Greenlandic waters and form blooms.

Aim and main questions: In this WP, competitive abilities of harmful and non-harmful microalgae will be assessed and compared under different scenarios, including the influence of grazing pressure, yielding a basis for prediction of species dominance. Parameters like efficiency of resource use (i.e. nutrients, light) and grazing defence or evasion (e.g. colony formation) are generally important, while some are specific to only harmful species, such as chemical components against competitors and grazers. If different organisms have different tolerance ranges towards rapid warming and seawater freshening, changes in those may further complicate the final species composition. We therefore want to mechanistically understand if and under which conditions harmful species can invade plankton communities in Greenland waters and may form blooms.

Approach: We will use key harmful and non-harmful phytoplankton species in experiments of different complexity settings in order to: a) mechanistically quantify competitive abilities and interactions using multiple single strain cultures of field isolates (see WP2) in the laboratory, (b) assess impact of grazers (copepod Calanus finmarchicus and fish larvae), and c) test invasive capacity of harmful and non-harmful species in natural communities under different scenarios (salinity, temperature, grazer) in Greenland (WP5). Experiments on competitive abilities and interactions will be repeated with isolates evolved under future conditions (see WP2). We aim to resolve which processes facilitate and hinder invasion and to apply this knowledge to define tipping points for the bloom dynamics and composition as it is currently known.

Output and deliverables: 1) growth rates, competition coefficients, nutrient uptake, toxin production/effectiveness (-> laboratory); 2) grazing rates, selective grazing, outcomes of more realistic invasion scenarios (->field) 3) model on using R* and/or competition coefficients which come from two species pairs to predict invasive capacity of larger communities under different temperature conditions.