Fig 1. Schematic representation of the impact of nutrient distribution on the opportunities for an opportunistic pathogen to invade the preexisting microbial community in an aquaculture system (matured water).

The darker the colour, the higher the nutrient concentration. (A) In the hypothetical scenario in which nutrients are homogeneously distributed in the water column, K-strategists are capable of using the nutrients, and there is no opportunity for the opportunistic pathogen to reach a high population density. (B) In reality, nutrients are not homogeneously distributed but rather present as hot spots with locally high nutrient concentrations. The nonmotile K-strategists remain randomly distributed and are not able to adequately exploit the nutrient source. Chemotactic motility enables the opportunistic pathogen to localise and exploit the hot spot, and as a consequence, its population density increases considerably. (C) If the original microbial community of the matured water also contains r-strategists, then these r-strategists can compete with opportunistic pathogens for the resources present in the hot spots. As a consequence, the opportunistic pathogen is not able to reach a density as high as in panel B. (D) If feed particles are used that disintegrate rapidly, then the time window during which the hot spot exists is limited and nutrients quickly diffuse into the bulk, thereby avoiding (locally) high levels of dissolved nutrients. As a consequence, K-strategists will be capable of utilising a significant fraction of the nutrients, and the opportunistic pathogen is not able to reach a density as high as in panel B.