Are you ready to discover another ecosystem straddling sea and land? In this second article of the column on transitional waters, Worldrise will accompany you on a journey through coastal lagoons.
What does the term “lagoons” mean?
According to the definition given by the encyclopedia Treccani, the lagoon is a “coastal water basin, separated from the sea by a coastal cordon interrupted by inlets, and in which island formations often emerge”. More simply, the term “lagoon” means a body of brackish water of marine origin, separated from the open sea by islands or barriers parallel to the coastline, formed as a result of the progressive accumulation of material due to the particular hydrodynamic motion that can be observed in this context.
As one of the most widespread transitional ecosystems in the world, lagoons are frequent along generally low coastlines, often near the mouths of large rivers, and usually affected by sensitive tidal excursions. They make up more than 10% of the world’s coastline! Along the Mediterranean Sea, there are hundreds of coastal lagoons, for a total area of 6500 km2, and many of them are located in Italy: the lagoons of Orbetello, Lesina, Venice, Varano, Marsala, Faro and Ganzirri, Sabaudia and Cabras are quite famous.
KEY ELEMENTS AND ZONATION
The lagoon is composed by:
- a freshwater input from the river,
- the presence of coastal cordons or islands, consisting of deposits resulting from solid input of river origin or coastal erosion, which make the basin semi-closed, separating it from the sea;
- the area of the vents, where there is the exchange between the external seawater and the internal brackish water (the absence of this element leads to the formation of coastal lakes and ponds),
- the presence of a drainage basin, through which water enters the soil.
A fine balance is established between the speed of rising sea level, the amount of terrigenous sandy contributions, the transport of sediment along the shores and climatic conditions, to ensure the lagoon system’s presence and proper functioning.
The lagoon environment is part of the so-called paralytic domain, a concept introduced by Perthuisot J.P. and Guelorget O. in 1992. This area can be divided into 6 zones, through which the progressive transition from the freshwater environment to the more strictly marine one is observed. In fact, as it happens in other transitional environments, such as salt marshes, also in the case of lagoons there are large and irregular spatial and temporal fluctuations of chemical-physical variables, which determine remarkable biodiversity and a system of zonation.
In general, the biomass and the biodiversity of the benthic communities increase proportionally to the salinity: from the rich meadows of marine seagrasses (e.g. Posidonia oceanica) dominated by stenohaline species, such as the star Asterina gibbosa, the sea cucumber Holothuria polii and the sea urchin Paracentrotus lividus, you move to a fauna able to survive and/or thrive in brackish waters (e.g. the clam Ruditapes decussatus and the polychete Hediste diversicolor), up to organisms typical of freshwater, like the insects’ larvae belonging to the family Chironomidae.
WHAT ARE THE TYPICAL FEATURES OF A LAGOON?
In general, lagoons are characterized by:
- large spatio-temporal variations in salinity, related to evaporation, precipitation, and river runoff;
- strong seasonal thermal excursion, which determines an annual cyclicity in the communities of macroinvertebrates living on the bottom;
- reduced hydrodynamism and possible stratification of the water column;
- vertical mixing, mainly due to the action of the wind, given the low depth of the lagoon basin;
- continuous intakes of detritus, organic matter and inorganic nutrients;
- high primary production, through both phytoplankton (mainly diatoms and cyanobacteria) and phytobenthos (macrophytes and macroalgae).
As we have previously seen for the marsh grasses in the salt marshes, the marine seagrasses present in the lagoons, such as species belonging to the genera Zostera and Ruppia, are not grazed by the aquatic organisms because they are rich in tannins and, therefore, difficult to digest. These plants, however, become part of the detritus food chain: in fact, lagoons behave like real “detritus traps“. Actually, the organic residues present in the lagoon waters are shredded and modified by detritivorous organisms, gradually becoming attackable by decomposers, such as bacteria and fungi, which re-mineralize them (hence they degrade the organic substances coming from living beings until their complete transformation into simple inorganic compounds), putting back in circulation the primary constituents (the minerals), that return to be available for the autotrophs (algae and plants). Then, bacteria and fungi become part of the classical trophic chain and will be predated by heterotrophic nanoflagellates, like protozoa, and meiofauna (small benthic invertebrates, such as nematodes).
The role of resuspension in the lagoon is very important because it facilitates the recycling of nutrients, promotes exchanges between water and sediment, supplies the plankton with organic particles, and allows the export to adjacent systems.
ECOSYSTEM SERVICES AND THREATS
Due to their high productivity, lagoons are considered to be historical withdrawal sites.
These ecosystems host the reproducers and the juvenile stages of some species of commercial interest such as sea breams, sea basses and mullets. Generally, traditional fishing methods and extensive aquaculture are used here, like gill nets, which convey the fish to a final net, bivalve mollusks farming (e.g., clams and mussels), shellfish fishing with dredges and, in the delta valleys of Po and Comacchio, the “lavorieri”, fixed installations aiming the capture of adult fish which, from the lagoon or the mouths of rivers, return to the sea in order to reproduce.
Being semi-enclosed basins, the lagoons are easily subject to eutrophication phenomena: the run-off from agriculture and the fluvial contribution constitute a high input of nutrients and organic matter, whose anaerobic degradation will lead to the consumption of oxygen and to the production of CO2.
Furthermore, if eutrophication occurs, the phanerogams are replaced by massive macroalgal beds, such as those formed by Ulva spp. (nitrophilic species, usually found in environments with high nitrate concentrations) or Enteromorpha spp. During the vegetative period, these species initially live suspended in the water column, subsequently settling at the water-sediment interface at the beginning of summer. The layer they create will undergo decomposition, leading to possible hypoxia/anoxia phenomena.
Further unfavourable conditions, such as the intrinsic slow water exchange of the lagoon system and the high temperatures of the water masses, can contribute to the determination of massive death events of benthic organisms, such as bivalves, or of the entire fish fauna.
THE CURRENT STATUS OF PROTECTION
The conservation of these habitats largely depends on the assessment of their natural characteristics and, in particular, on their biodiversity, which is one of the main criteria used for the development of wetland protection policies (Ramsar Convention Bureau, 2005). For example, the composition and abundance of benthic invertebrates are two of the most important elements to take into consideration, as well as the presence of insects belonging to the Coleoptera and Hemiptera orders, two of the most common groups in these environments and remarkably important as far as it concerns the food chain of freshwater environments.
At the European Union level, the protection of these ecosystems has been designated as a priority by the Habitat Directive 92/43/EEC in Annex I (“natural habitat types of community interest whose conservation requires the designation of special areas of conservation”). 771 of these habitats have been included in the Natura 2000 network, which is an ecological network aiming to promote restoration and protection projects, in compliance with the objectives set out in the Habitats Directive.
Fjords and rias represent another kind of transitional waters, but we will discuss this topic in the next article of the column. Are you curious to discover the biodiversity hidden within these wonderful ecosystems?
Bibliography and sitography:
- Danovaro, R. (2019). Biologia marina: Biodiversità e funzionamento degli ecosistemi marini. Seconda edizione. UTET università.