Effects of changing environmental conditions on plastic ingestion and feeding ecology of a benthopelagic fish (Gadus morhua) and its prey (Sprattus sprattus) in the Southwest Baltic Sea

Plastic pollution is ubiquitous to all marine environments. As such, monitoring the distribution of plastic within the water column and the ingestion of plastic particles by marine organisms is crucial to understanding the risks posed by plastic to the marine ecosystem. Within the Baltic Sea, this anthropogenic stressor is additional to naturally occurring stressors, including anoxia at depth in years with decreased inflow of high saline oxygenated water from the North Sea. This study documents the occurrence of microplastics (< 5 mm) ingested by Atlantic cod (Gadus morhua) and their prey, European sprat (Sprattus sprattus), within the Bornholm Basin (Southwest Baltic Sea) in relation to the vertical distribution of plastic within the water column. It was expected that in years following low inflow events, anoxia at depth would cause cod to move upwards into oxygenated areas within the water column. These depths were likely to contain a higher abundance of microplastics, which in combination with a shift in cod feeding strategies to a majority pelagic diet, would result in an increase in plastic ingestion through trophic transfer from pelagic prey. Sprat were expected to be found within the majority of the water column regardless of anoxia so their plastic ingestion was not expected to change with regard to inflow level. MultiNet samples (n = 20), were collected annually between 2014 and 2018 at shallow (20–25 m), intermediate (65–70 m), near-bottom (85–90 m) and halocline depths, and analyzed to quantify microplastics within the water column. The gut of fish (cod n = 72 and sprat n = 50) collected under different Major Baltic Inflow levels of varying oxygen, salinity and temperature conditions between 62 and 87 m depth were analyzed for ingestion of microplastics and gut content. Plastic abundance was found to be homogenously distributed throughout the water column, and was not influenced by the inflow events. Under anoxic conditions, the majority of Atlantic cod (81%) fed exclusively on pelagic prey (sprat) whereas, under oxygenated conditions, the majority of cod fed on benthic invertebrates indicating benthic (61%; exclusively benthic invertebrates) or benthopelagic feeding (13%; fish and invertebrates). The proportion of cod that had ingested microplastics was higher under anoxic than oxygenated conditions (38% and 15% of the fish total, respectively). This is due to the upwards shift in foraging area within the water column, where principally feeding on pelagic prey led to an accumulation of microplastics through trophic transfer. The ingestion of microplastics by the sprat did not increase under anoxic conditions. As Baltic trends point towards an increase in anoxia at depth, microplastic ingestion by Atlantic cod, through trophic transfer from their prey, e.g. European sprat, is expected to increase in the future. The potential impacts of microplastics on Atlantic cod warrant to be examined in future studies to evaluate the risks posed by these contaminants on the natural population of one of the most important commercial species within the Baltic.