In the food webs of the Baltic Sea, energy is transferred from plankton to the apex predators

Food webs describe how energy flows from one level of the food web to another in an ecosystem. In other words, who eats whom or who eats what and how much.

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Riikka Puntila

The writer works in the marine ecosystem modeling group of the Finnish Environment Institute, studying the functioning of the Baltic Sea food webs in nature and through ecosystem modeling.

The food web can be described as a network-like structure of many food chains, with nodes for different species or species groups. The food web informs us about the relationships between the species of an ecosystem concerning their food consumption and predation.

The productivity of an ecosystem as a whole depends on the functioning of food webs, as most products of the sea, such as fish, prawns and shrimp, are consumers. In other words, they are on the higher levels of the food web and are thus dependent on producers, grazers, as well as intermediate consumers.

 Tiny copepods have two pairs of antennae and a forked tail with lots of bristles.
Copepods are an important food source for juvenile fish.

The food webs of the Baltic Sea are divided into three zones

The Baltic Sea ecosystem and its functioning food webs can be divided into coastal zones, deep-sea bottoms, and open-sea zones. However, these different zones are constantly interacting with each other because many species move between them, either continuously or at different times of the year.

A particularly clear example of such interactions can be found in deep-sea bottoms. In the absence of light, primary production is minimal or non-existent, thus the energy of the deep-sea seafloor comes from the surface layer of the open-sea zone.

Primary production occurs when plants produce energy through the process known as photosynthesis.

Food webs are based on primary production occurring either in the water body or from an external source

In all ecosystems, the foundation of each food web is primary production, which varies, among other things, with water depth, bottom quality, and water currents. Primary production can occur either in the water body itself or derive from an external source. 

A water body’s primary producer organisms bind light energy from the sun, converting it to organic carbon through photosynthesis. In this case, the total amount of basic production depends not only on the level of nutrients used but also on the seasonal variation in the amount of light.

These primary producers can include small planktonic algae living in the water column or on the seabed, soft bottom vegetation or macrophyte algae growing on hard bottoms.

In the open seas, the main group of primary producers is phytoplankton, which accounts for 90% of production. Other important primary producers in the Baltic Sea include the vascular plants of shallow areas, as well as the macro-algae of hard rocky shores.

 A group of brownish pond snails graze on a rock surface.
Pond snails eat, among other things, algae from rock surfaces.

Organic matter may also enter water bodies from external sources. Substances carried in river water and runoff are often very fine-grained or form dissolved compounds in water. Bacteria can use such organic material as a food source.

These tiny bacteria are eaten by dinoflagellate organisms, which are also invisible to the naked eye. In turn, dinoflagellates end up being eaten by ciliate organisms. Thus, the organic material discharged into the water body only reaches a suitable size for zooplankton after these steps have occurred.

Thus, energy must pass through several steps before becoming food for zooplankton. This will result in a long food chain and a significant loss of energy. Therefore, a microbial loop based on bacterial production, i.e. a micro food chain, is quite inefficient in transferring energy to the upper stages of the food web.

However, the microbial loop is the most important route through which the dissolved organic matter can be returned to the ecosystem. At times, microbial production is very active, and its significance is by no means negligible.

All levels of the food web depend on primary production

Primary production is regulated by light and its specific discharge and is used by all consumers, either directly or indirectly.

The first-level consumers, i.e. grazers, feed on the primary producers. At the lowest level of the production network, the size variation is large. There are all sorts of organisms, from bacteria to macro-algae, such as bladder wrack. As a result, the next grazing level is highly diverse.

In the open seas, grazers include zooplankton or mussels, while in the coastal zones, they consist of invertebrates and fish, which feed on algae and macroalgae. In time, grazers are eventually eaten by second-level consumers, i.e. predators.

When they die, the organic matter from uneaten organisms is consumed by decomposers and ends up back at the bottom of the food web, from where it is put back into the whole cycle again.

The highest levels of the food web are represented by apex predators, such as marine mammals, birds of prey, or by humans fishing or hunting.

A shoal of small fish swim over an impenetrable meadow of watermilfoil.
Fish search for food in the lush jungles of Siberian watermilfoil.

The food webs of the Baltic Sea are quite simple

In the Baltic Sea, the food webs are typically simpler than those in the oceans because there are fewer species. Nevertheless, the productivity of the Baltic Sea is very much in proportion to that of the oceans.

On the other hand, a simpler ecosystem may be more sensitive to environmental changes, and each species is important for that ecosystem to function. Moreover, in a simple food web, the transfer of energy is more efficient the fewer steps it has to travel.

In other words, the more levels there are in a food web, the more primary production is needed to support the production of higher levels.

Human activities and alien species have changed the food webs of the Baltic Sea

The food webs of the Baltic Sea have constantly been changing throughout their history. These changes have been caused, among other things, by the introduction of new species, as well as by variations in water salinity. The result is the current fragile ecosystem, the operation of which is easily disrupted.

Changes have also taken place in the fish community. Cod populations have declined due to fishing, as well as the deterioration of conditions suitable for reproduction. Conversely, the stocks of sprat, an important prey food of cod, have markedly increased. 

Herring and sprat compete for some of the same food sources. Indeed, in the main basin of the Baltic Sea, herring stocks and especially herring growth have declined.

Apart from humans, the number of top predators in the food web has decreased. As a result, the functioning of the entire ecosystem has changed radically. In recent decades, dozens of new alien species have been introduced to the Baltic Sea, which has contributed more to the transformation of the traditional food webs.

The EU monitors the state of the Baltic Sea food webs

The importance of maintaining properly functioning marine food webs has been recognised, and the Marine Strategy Directive of the European Union (EU) monitors their status with the help of indicators.

According to EU definitions, a good condition status of food webs is when "all components of marine food webs, as far as they are known, are present in their normal abundance and diversity and at levels that ensure the long-term abundance of species and their reproductive capacity."