2.3 A complex biodiversity

Mediterranean coastal lagoons are unanimously described as areas exposed to natural disturbance, depending on morphodynamics and climatic factors such as freshwater flooding and summer drought. This brings about fluctuations in salinity, which are due to seasonal variations in precipitations and evaporation, and marked seasonal temperature fluctuations, 1
which are more or less significant depending on the lagoon size and latitude. These oscillations 3
may create extreme conditions for many taxa, and the resulting biodiversity is characterized by a low number of highly specialized species. On the other hand, the composite mosaic constituted by the surrounding lagoon landscape, barrier islands, coastal spits and contiguous wetlands provide ecotones which create extremely diversified habitats supporting a rich biodiversity.

2.3.1 Seagrasses and seaweeds

A key factor in sustaining lagoon biodiversity is the presence

of seagrass meadows that proliferate in productive lagoons with an efficient trophic transfer; seagrass meadows are in fact at the basis of the ecosystem functioning and create the potential for specific habitats. In the Mediterranean region, there are three species of euryhaline seagrasses: Zostera noltii, Z. marina and Cymodocea nodosa. These seagrass species not only provide the physical habitat for a rich fauna but also play a fundamental role in biogeochemical processes contributing to lagoons water quality. With low nutrient levels and clear water conditions, such as those typical of a pristine oligotrophic state, Zostera spp. and the aquatic plants of the Ruppia genus are the dominant macrophytes taxa of the lagoon. By contrast, the massive presence of opportunistic seaweeds such as Gracilaria, Ulva and Cladophora spp. along with cyanobacteria indicate a degraded eutrophic state of the lagoon environment (Viaroli et al., 2008).

When seaweeds constitute an important biomass, they can be exploited with conspicuous yields and incomes, as it occurred in the Nador lagoon, in Morocco (Morocco country report) and in the north Adriatic lagoons, where green algae and Gracilaria are used for the production of paper and manure for agriculture as well as in the chemical and
pharmaceutical industries (Italy country report).

 

2.3.2 Benthos

Lagoon benthos includes a wide variety of mobile or sessile aquatic organisms living in or on the substrate and represents a cornerstone for the cycle of nutrients in the lagoon environment. With regard to photosynthetic organisms, besides seagrasses and seaweeds as mentioned above, a large number of microscopic primary producers (microalgae or phytoplankton) can grow attached to solid substrates, on rocks, on the sediment or on macrovegetation. Zoobenthos consists of macroinvertebrates that live on the bottom of water bodies and on some macrophytes. In addition to molluscs (filter feeders) and crustaceans, which are very important for lagoon fisheries (see paragraph 2.4.1), lagoon zoobenthos includes flatworms, annelids and larvae of insect species that can tolerate salinity variations. The surface area, water salinity and outlet width and length of lagoons can actually be considered as key parameters limiting and defining the environmental niche space of benthic macroinvertebrates in lagoon ecosystems (Garrido et al., 2011).

2.3.3 Fish communities

Coastal lagoons are nursery areas, feeding grounds and pathways for the diadromous6 migration of many fish species (Koutrakis et al., 2005; Franco et al., 2006a). Regardless of morphological and environmental variability related to the size of the lagoons, the gradients and the trophic state of the waterbody, the basic structure of fish communities in the Mediterranean does not differ considerably from one lagoon to another; it shows a substantial stability characterized by a relatively low diversity in species but a high abundance of specimen per taxon. The structure of fish assemblages can be attributed to the dominance of those groups that better tolerate the fluctuations of abiotic and biotic factors as well as to the nature of food webs that are established in the lagoon systems.
When considering fish species in relation to their ecological guilds (Elliott and Dewailly, 1995), different groups are represented in the Mediterranean: from resident species that spend their entire life cycle within the lagoon, to marine seasonal species that reproduce at sea but migrate to lagoons for trophic reasons and marine precocious juveniles migrants that use lagoons as nursery areas. Adventitious marine species that occasionally enter lagoons are generally present near the tidal channels, while freshwater species usually remain close to river inputs. Species richness and composition of fish communities can be explained by the rate of water exchange between the sea and the lagoon (Perez-Ruzafa et al., 2007), which supports the idea that fish species richness in coastal lagoons is mostly determined by colonization rates from the adjacent coastal zone (Mariani, 2001; Perez Ruzafa et al., 2004; 2006). Franco et al. (2008) has confirmed that, in Mediterranean coastal lagoons, fish assemblages were dominated on average by marine species with a majority of marine migrants (juveniles and adults) rather than marine stragglers.

 

Fish harvest from the Tortoli lagoon (Sardinia, Italy), photo ©F. Massa, 2011

6 Migration of fish between the sea and freshwater.

Species such as Anguilla anguilla, Atherina boyeri, Dicentrarchus labrax, Liza ramada, Mugil cephalus and Sparus auratus, have been reported to be present in all Mediterranean lagoons; other species of Mugilidae and Sparidae have been found very frequently in more than 70 percent of the lagoons, while about a hundred species have been characterized as occasional or specific to few lagoons since they were present in less than 15 percent of the lagoons (Perez Ruzafa et al., 2007).
Concerning the European eel (A. anguilla), it is worth noting that it is the only catadromous species of the Mediterranean which migrates over long distances between the ocean – where eels spawn – and brackish and freshwaters – where growing takes place. The complex biology and life cycle of eel makes it exposed to several threats of anthropogenic nature, including
overfishing, pollution, coastal habitat loss and climate change. These are supposedly the main causative agents of the stock-wide decline that this species has been facing in Mediterranean lagoons and in European inland and coastal waters. A comprehensive review on European eel is presented in case study 3 (see paragraph 3.3) and specific issues related the management of eel stocks are illustrated in case study 4 (see paragraph 3.4).
Besides the above-mentioned species, a number of adventitious coastal marine species can be found in Mediterranean coastal lagoons. They occasionally enter in lagoons whenever they find favourable conditions (e.g. reduction of freshwater inputs that can entail a rise in salinity or a lessening of saline gradient) and remain for long or short periods. Sardines, croakers, seabreams, red mullets, flounders, soles, groupers can be occasionally or permanently present within the lagoon fish assemblage and become a conspicuous share of the lagoon yields. In recent years, also lessepsian species such as the marbled spinefoot (Siganus rivulatus) and the dusky rabbitfish (S. fuscescens) have entered in the Mediterranean basin via the Suez Canal and have been commonly reported as dwellers of eastern Mediterranean lagoons.
In coastal lagoons with low salinity, freshwater species such as tilapias (Oreochromis niloticus and Sarotherodon galilaeus) and cyprinids (Cyprinus carpio and Carassius carassius) can also be present (Egypt and Spain country reports). The big-scale sand smelt (Atherina boyeri) is the only 1
commercial euryhaline finfish that reproduces in coastal lagoons. A. boyeri has been subject to 5
several systematic revisions and genetic studies, which indicate the existence of three species belonging to the genus Atherina: boyeri, lagunae (Milana et al., 2012) and mochon present in the Manzala lake, Egypt (Egypt country report).
The presence and persistence of adventitious marine species can be the symptom of a change in the lagoon conditions, as indicated by the occurrence of anchovy (Engraulis encrasicolus) in the Nador lagoon, in Morocco (Malouli Idrissi and Zahri, 2002; Malouli Idrissi et al, 2003; Morocco country report) and in the lagoon complex of the Valli di Comacchio, in northern Italy (Italy country report) where anchovies are caught together with sprat (Sprattus sprattus).

2.3.4 Birds

The number of aquatic bird species observed in Mediterranean lagoons adds up to several hundreds, given that the Mediterranean region is an important refuge of migrating birds from southern and northern latitudes. The richness and population abundance of avian species indicates that the aquatic bird fauna is extremely important in terms of ecological relevance and conservation in all the lagoons of the Mediterranean. Birds are in fact critical components of the lagoon ecological processes (e.g. food webs) and, at the same time, coastal lagoons are essential for all wetland birds as they provide habitats and feeding grounds. Lagoon ecosystems can sustain both herbivorous (e.g. waterfowl of the Anseriformes order, ducks, geese, swans, etc.) and carnivorous species that feed on small benthic invertebrates and on fish (e.g. cormorants).
Water birds are probably the most extensively monitored species among the living components of coastal lagoons (Aymerich and Cedran, 2011) and several international legal instruments have been promoted to protect them, starting with the Ramsar Convention (1971), which is the first intergovernmental treaty providing a framework for national action and international cooperation for the conservation and wise use of wetlands and birds.

Common stilt (Himantopus himantopus) specimen in the Cabras lagoon (Sardinia, Italy), photo ©A. Santangeli, 2008
Recently, the increasing number of ichthyophagous birds inhabiting coastal lagoons has created 1
a remarkable impact on lagoon fishing production; conflicts between fishers and bird 6
conservationists are still ongoing (see paragraph 2.6.5).