Copepods are small, shrimp-like crustaceans that inhabit nearly every aquatic environment, forming a major component of the zooplankton community and serving as a primary food source for many fish and invertebrates. Cyanobacteria, often called blue-green algae, are photosynthetic bacteria that can proliferate rapidly to form visible blooms, which are frequently harmful due to the production of potent toxins. The relationship between these two groups is a central topic in aquatic ecology because the copepod’s feeding habits significantly influence the fate of cyanobacteria blooms. This trophic link involves intricate behavioral, chemical, and physical interactions that determine whether these bacteria are consumed or avoided. Understanding this dynamic is a major focus of current research aimed at managing water quality worldwide.
Selective Feeding and Avoidance Strategies
Copepods are not indiscriminate grazers; they exhibit highly selective feeding behavior, often preferring eukaryotic algae over cyanobacteria. This preference is a crucial determinant of their impact on the aquatic environment, as they actively choose the most nutritious and edible food particles available. Some copepod species, such as certain Calanus or Notodiaptomus species, may consume cyanobacteria, especially when other, higher-quality food sources are scarce or at low concentrations.
Most copepods, however, employ sophisticated avoidance strategies when encountering cyanobacteria, recognizing them as low-quality or potentially toxic prey. Calanoid copepods, for instance, can use chemosensory cues to sense chemical signals, including toxins, allowing them to reject the particle before ingestion. This behavioral response is often observed as an alteration of their feeding current or swimming pattern, effectively flicking away the unwanted cyanobacterial cells or filaments. By maintaining uninhibited ingestion of alternative prey while avoiding the cyanobacteria, selective copepods can co-exist with dense blooms. This selective grazing can sometimes indirectly facilitate cyanobacteria dominance by eliminating their more edible phytoplankton competitors.
Key Factors Influencing Consumption
The decision by a copepod to consume or reject cyanobacteria is governed by a combination of physical, chemical, and nutritional factors.
Chemical Factors
One of the most significant chemical drivers is the presence of cyanotoxins, such as microcystins or nodularin. Higher concentrations of these toxins often lead to a strong rejection response by the copepods, resulting in a reduction in their filtration and ingestion rates. However, some copepods have evolved mechanisms, including gut bacteria carrying the mlrA gene involved in toxin degradation, that may allow them to cope better with a cyanobacteria-containing diet.
Morphological Factors
The morphology and size of the cyanobacteria also play a major role, often acting as a physical deterrent. Filamentous or colonial cyanobacteria, such as Microcystis colonies or long Cylindrospermopsis raciborskii filaments, are physically difficult for many small copepods to handle and ingest. Conversely, small, single-celled cyanobacteria are more likely to be consumed, provided they are not highly toxic. In some cases, the negative effect of toxicity is so pronounced that it overrides the morphological barrier, meaning a highly toxic strain is avoided even if it is a short filament.
Nutritional Factors
The nutritional quality of the food is another major factor, as cyanobacteria are often considered a poor food source compared to eukaryotic algae. Cyanobacteria generally have a lower concentration of highly unsaturated fatty acids and are sometimes nitrogen-poor, which are essential for copepod growth and reproduction. This low nutritional value, coupled with the potential for toxicity, influences the copepod’s feeding decision, leading to a strong preference for more nutritious prey. If the cyanobacteria are the only available food, or if the copepod’s gut microbiota can mitigate the toxicity, the nutritional deficit may be temporarily overcome.
Consequences for the Aquatic Food Web
The selective feeding and frequent avoidance of cyanobacteria by copepods have profound consequences for the entire aquatic food web structure and function. When cyanobacteria dominate the primary producer community, they create a “trophic bottleneck” that inhibits the efficient transfer of energy up the food chain. Since copepods avoid or poorly utilize this energy source, less carbon and biomass are transferred to higher trophic levels, such as fish and birds. This reduced energy flow can impact the overall productivity of the ecosystem.
Even when copepods consume small amounts of toxic cyanobacteria, there is a risk of toxin transfer and bioaccumulation through the food web. Toxins like microcystins and nodularin can accumulate in the copepods’ tissues and then be passed on to their predators. This bioaccumulation poses potential health risks to organisms at higher trophic levels, including commercially important fish and shellfish, and ultimately to humans who consume them.
The avoidance behavior of copepods can contribute directly to the persistence and intensification of harmful algal blooms (HABs). If the primary consumers, the copepods, do not effectively graze on the cyanobacteria, they fail to act as a natural top-down control mechanism on the population. This lack of grazing pressure allows the cyanobacterial populations to continue growing unchecked, leading to denser, more prolonged, and potentially more toxic blooms in the water body.