The editors*

1. Hydrochemical characteristics and trophic status

1Time and space variability of hydrologic and trophic characteristics of the lagoon were described in detail by Esquivel & Soto-Castor, and by Aguirre-León et al. and Figueroa-Torres et al. (this issue). They were also briefly documented in some other chapters. All studies agreed that Sontecomapan Lagoon can be divided into three main “habitat zones”, mainly driven by their spatial and temporal exposition to tidal exchanges with the sea, freshwater inputs, their morphology and their isolation degree, these are:

• an euhaline zone, in the northern part between La Palma River and the communication with the Sea, characterized by the highest marine influence.

• a mesohaline zone located in the southern inner part of the lagoon with highest freshwater influence.

• an intermediate polyhaline zone with combined effects of marine and freshwater influence.

2The contours of these three zones vary seasonally with an extension of the mesohaline zone in the rainy season. Also, Figueroa-Torres et al. (this issue) point out that the area near Río Sábalo, which corresponds to the more confined zone, clearly distinguished from all other zones in all seasons.

3Esquivel & Soto-Castor (this issue) show that the water column is generally well mixed and poorly stratified in most part of the lagoon, except in the area of La Palma due to the combined influence of tide and river flow provoking episodic salt wedge situations.

4The trophic status of the lagoon varies spatially and temporarily. Based on total phosphorous indexes, the lagoon may be considered as mesotrophic to hypertrophic whereas it ranges from oligotrophic to eutrophic according to chlorophyll a values (Esquivel & Soto-Castor; this issue). Inter-annual variations of trophic status are pointed by Esquivel & Soto-Castor (this issue). By comparing historical chlorophyll a values they show that values for 2009-2010 were the lowest for the period 1990-2010, whereas N-nutrients were within the ranges and soluble phosphorous (SRP) was at higher concentrations than values reported for the same period. They hypothesize that the diminution of chlorophyll a, was probably related to exceptional rainfall during this period, especially during their September survey, which probably caused a flushing out of suspended materials to the sea.

2. Living communities - taxonomic richness

5Castellanos-Páez et al. (this issue) reviewed about 59 papers between scientific articles and book chapters published between 1976 and 2015 and updated the checklists of the different biota of Sontecomapan lagoon. They could establish several species checklists.

• The list of flora (phytoplankton and plants) includes 431 species, where 403 are phytoplankters, 4 macroalgae, 1 seagrass, 3 mangrove, 7 species of coastal vegetation, 1 fern, 2 legumes, 1 liana, 2 bromeliads, and 7 orchids. The list of phytoplankton was also updated by Figueroa Torres et al. (this issue) who reported 357 species from a review of the literature and samples collected by them in 2015/2016.

• The list of invertebrate species has a total of 201 species of which are: 5 platyhelminthes, 6 trematodes, 2 nematods, 69 rotifers, 8 acantocephalans, 69 molluscs and 39 crustaceans (1 pentastomid, 3 cladocerans, 14 copepods, 1 rhizocephalan,
  1 tanaidacean and 19 decapods), 3 insects (odonata).

• The list of vertebrates represents a total of 185 species of which are: 159 birds, 14 fishes hosting parasites, 1 amphibian, 8 reptiles and 3 mammals. In another paper of this issue, Castillo-Rivera and Lara-Domínguez report 115 fish species in Sontecomapan, of which 4 species are not included in the list of the above14 species hosting parasites: Diapterus olisthostomus, Lujtanus cyanopterus, Trachinotus carolinus and Scomberomorus maculatus. Thus, the actual list should include 119 fish species and the total number of vertebrate species would be 294.

6Taking account of these different contributions, the current number of living species recorded in the Sontecomapan ecosystem is 926.

3. Ecological functioning

7From different contributions of this book, we could set a preliminary conceptual model of the trophic flows in the Sontecomapan Lagoon (Figure 1).

Figure 1. Conceptual model of trophic flows in the Sontecomapan Lagoon. In red characters are the biotic compartments not treated in this monograph. Blue arrows illustrate the trophic flows. Black arrows indicate recycling process (excretion). Orange arrows illustrate inhibitory relationships.

8The Figure 1 illustrates the main pathways between the different trophic compartments identified for this model.

9The main driving forces of the systems are rainfall (and associated runoff) and tidal exchange which condition the C, N and P equilibrium in the lagoon. The balance between these forces is important, along with wind stress that determine the complete mixing up of water column at most of the lagoon and are responsible for the changing horizontal zonation at different seasons. A further driving force comes from water discharge or runoff from human settlements or other anthropic activities. In non-disturbed conditions, P enrichment should occur from seabird droppings at their perching sites in mangroves and coastal fish P-excretion. Recycling by metazoans which may be several fold higher than that of bacteria (Kremer & Nixon 2012) is an important process to be considered, but data are lacking to estimate the importance of these processes in Sontecomapan lagoon. Mangroves also appear to have a very important role (see below) and Hernández-Gaona et al. (this issue) also point out to mangrove area loss as a factor determining the possible Sontecomapan’s future states. As in many shallow water systems, the primary production of the lagoon is shared between phytoplankton, floating and fixed macrophytes, mangrove and phytobentos (not shown). In such ecosystem, the balance between these different types of primary producers for the use of nutrients is important for controlling eutrophication. Generally, the importance of macrophytes which compete with phytoplankton for nutrients is a guarantee against the deterioration of water quality and shift between alternative stable states, i.e. between clearwater and turbid water (Scheffer et al., 2001). Also plant beds host macrofiltrators such as mussels and microfiltrators, which may help reducing turbidity of the water within the plant beds.

10However, for Sontecomapan, information on macrophytes and phytobentos is lacking. Indirect information by González-Fierro and Ponce-Vélez (this issue), suggests that the seagrass (Ruppia maritima) should be important. Invertebrates should also play an important role as the mussels (Brachidontes exustus), the oyster (Crassotrea virginica), the white shrimp (Litopenaeus vennamei), the lobster (Machrobrachium rosenbergii) which have commercial interest. Another important issue is to estimate matter export through fishing and birds. Exportation of matter by migrating bird has been shown to be a very important process in lakes or coastal lagoon. For example in the Ichkeul lagoon (Tunisia) where the macrophyte biomass is consumed by the herbivorous wintering waterfowl between October and March, the rather atypical, oligotrophy seems to be strongly linked to the exportation of the nutrients by the migrating birds at the end of the winter (Casagranda et al., 2006).

11Despite the lack of information on several trophic compartments, our results highlight some important characteristics of the ecological functioning of the lagoon, as detailed on the following points

The strong variability of nutrient availability drives the plankton dynamics

12Nutrients concentrations in the lagoon are driven by autochtonous sources as the mangrove leaf, the water exchanges with the ocean and through the freshwater inputs of the small rivers (mainly La Palma River). Seasonal variation is important with phosphorus peak occurring in early rainy season (June) and nitrogen peak centered on September (late rainy season) (Esquivel & Soto-Castor, this issue). Negative relationship with oxidative forms of nitrogen (NO₃, NO₂) suggests important oxidization of NH₄ into NO_x. Another important source of dissolved mineral nitrogen is the decomposition of the mangrove litters (Aké-Castillo and Vázquez, this issue). At the water - sediment interface, the nutrient availability is also highly dependent on the bacterial activity which drive incorporation and loss processes of N species (i.e. to nitrogen fixation and denitrification) in the bottom water and in the superficial sediments (Ferrara-Guerrero et al., this issue). Due to the shallowness of the lagoon, these processes are highly dependent of climatic conditions. In “Nortes” season N₂ fixation is highest, due to optimal oxic conditions and high organic matter concentrations in the sediments resuspended by wind creating microhabitats with aerobic and anaerobic spaces favoring the bacterial processes. During the rainy season, low salinity and high NO₃ concentration favor the denitrification process.

13Then high spatial and temporal variability also drive the dynamics of phytoplankton communities (Figueroa Torres et al., this issue). During the rainy season freshwater inflow decreases the salinity and fertilizes the lagoon in nutrients stimulating new primary production and promoting a phytoplankton community dominated by the chlorophytes. In the dry season, high concentrations in ammonium and high bacterial biomass suggest remineralization processes favoring regenerated production leading to the development of a more diversified phytoplankton community characterized by chrysophytes, cryptophytes, diatoms and dinoflagellates. During the North-wind season high phosphate concentrations may explain the development of another phytoplankton community characterized by euglenophytes and prasinophytes. These seasonal changes in phytoplankton affect the composition and size-structure of zooplankton communities showing the structuring character of trophic conditions on these communities (Benítez-Díaz Mirón et al., 2014; Benítez-Díaz Mirón et al., this issue).

The mangrove plays an important ecological role but is threatened by Human activity

14The banks of the lagoon are covered with red (Rhizophora mangle) and black (Avicenia germinans) mangrove which may play an important role in the ecological functioning serving as refuge for many species (bird, fish, molluscs, crustaceans, etc.). In this monograph, the work by Aké Castillo and Vázquez (this issue) showed that the mangrove also play an important role in the cycle of biogenic elements. These authors refer that NH₄ and NO₃ are quickly released during the early stages of litter decomposition in relation with a rapid mineralization, contrarily to phosphorous form (PO₄). This suggests a stimulating and selective effect of mangrove litter on primary production. However, litter decomposition has also an antagonist negative impact on phytoplankton through release of folin phenol active substances (FPAS) which can inhibit the growth of some species, resulting in a selective impact on phytoplankton community. In particular, some species (Chaetoceros muelleri var. subsalsum, Cyclotella cryptica, and C. meneghiniana) were shown to be sensitive to high concentrations of FPAS. Skeletonema subsalsum was able to tolerate moderate concentrations of FPAS. These responses support the hypothesis that tolerance to organic compounds in natural systems influence the dynamics of phytoplankton communities.

15It was seen that the area of mangrove forest in the lagoon tends to be disturbed by anthropogenic factors, leading to a deterioration of the ecosystem. Hernández-Gaona et al. (this issue) using Landsat satellite imagery showed a decreasing trend of mangrove area linked to increase of agriculture zone. The annual loss of mangrove forest area (422 ha; 1.64 % per year between 1979 and 200) has also strong economic impact with a loss of approx. $ 4,194,000 USD ha^-1 year^-1.

Low food transfer efficiency at the basis of the food-web

16The very low mean zooplankton/phytoplankton biomass ratio, compared to literature data, reveals an overall low food-transfer at the basis of the food chain (Benítez-Díaz Mirón et al., this issue). This low transfer would be partially due to a high predation impact by zooplanktophagous on larger zooplankton (calanoid copepods) as suggested by the zooplankton size-structure dominated by small organisms. This high predation, that lowers the herbivorous biomass, seems particularly important during the north wind season perhaps in relation with the breeding cycles of important fish species as the catfish Cathorops aguadulce very abundant in the lagoon (Aguirre-León et al., this issue). The low food transfer between phyto- and zooplankton may also result from inedibility and/or toxicity of a large part of phytoplankton with large proportion of dinoflagellates and diatoms, of which several species may be toxic to zooplankton (Figueroa-Torres et al., this issue).

17Benítez-Díaz Mirón et al. (this issue) also show that bottom-up forces affect phytoplankton (NH₄ limitation) and zooplankton (phytoplankton limitation), whereas the top-down control by herbivorous zooplankton on phytoplankton is very low (<10 % of the phytoplankton stock day^-1). As a consequence, such low exploitation of the phytoplankton production by zooplankton, should foster the sedimentation of un-grazed dead cells and the accumulation of organic matter on the bottom that could lead to the silting up of the lagoon and anoxia events. This tendency, which is also favored by the use of fertilizers and pesticides and associated eutrophication, would cause a degradation of the ecosystem structure and functioning. However, the high rate of water exchange with the ocean and the resulting low water renewal time in the lagoon should preserve the lagoon from these degrading trends, with the exception of the confined areas that must be surveyed and monitored in priority. In addition, tidal inputs of large-sized marine coastal zooplankton (e.g. Paracalanus aculeatus, Temora turbinata and Centropages velificatus) may compensate the “erosion” of large herbivorous zooplankton in the region close to the communication channel thus lowering the negative effects of predation by fish.

Environmental drivers of living communities: important role of salinity

18As discussed above, space and time variation of environmental factors in the Sontecomapan Lagoon are driven by seasonal patterns and by spatial and temporal exposition to tidal exchanges with the sea and to freshwater inputs. All studies dealing with functional aspects of the living communities considered in this monograph (bacteria, phytoplankton, zooplankton, mangrove and fish) point out the essential role of salinity in driving their dynamics.

19As show by Ferrara-Guerrero et al. (this issue) salinity and nutrient availability (NO₃) condition the balance between N₂ fixation and denitrification processes driven by bacterial activity with, low salinity and high NO₃ favoring the denitrification.

20The dynamics of phytoplankton communities is also highly dependent on nutrient availability and salinity (Figueroa-Torres et al., this issue). High NO₃ concentration and low salinity favor the development of chlorophytes during the rainy season whereas high ammonium concentrations and high salinity promote a more diversified phytoplankton community during the dry season. However, other physical and chemical factors such as pH, temperature and dissolved oxygen may generate particular microenvironmental conditions driving the distribution and abundance of phytoplankton species in relation with their tolerance capacities.

21Nutrients and salinity may also play an indirect role in structuring the zooplankton community, through their impact on their food resource (phytoplankton), as the study by Benítez-Díaz Mirón et al. (this issue) shows the high dependence of zooplankton on food concentration. These authors could not evidence a direct link between salinity and zooplankton biomass composition and size structure, perhaps in relation with the euryhaline character of the dominant species (Acartia clausi). However, in a previous study (Benítez-Díaz Mirón et al., 2014), they showed that transparency, salinity, temperature pH, as well as food availability were the main determinants of zooplankton abundance, composition, and diversity, explaining the seasonal differences of their communities.

22The fish community was also shown to be highly dependent on salinity, temperature, dissolved oxygen, pH, depth and transparency which define their habitat and explains their distribution and migration patterns during their life cycles (Aguirre-León et al., this issue). Three fish communities were defined mainly based on the life cycle and tolerance to salinity of the species: Marine euryhaline (ME, 27 species), Marine stenohaline species (MES, 3 species) and estuarine-lagoon permanent inhabitant (E, 1 species.)

4. Contamination and potential sanitary problems

23As highlighted in different studies presented in this monograph, the Sontecomapan Lagoon is faced to several ecological and sanitary threats linked to the contamination of water and/or sediment in relation with human activity.

Chemical contamination

24Metal concentrations (Cr, Ni, Zn, Cu, Co, Cd and Pb) in sediments, submerged vegetation and bivalve molluscs were monitored during an annual cycle in 1991-1992 by González-Fierro and Ponce-Vélez (this issue), in the frame of a general evaluation of the contamination levels of the coastal systems of the Gulf of Mexico (González-Fierro, 1995). At that time, the Sontecomapan Lagoon was chosen as a reference (pristine) site due to low human activity around the site. Nevertheless, high levels of total and bioavailable concentration of metals in surface sediments were recorded at sites of large river discharges. In particular, Cr, Ni and Zn concentrations were particularly high and Ni presented a real ecological risk for benthic species. High concentrations of Cu and Zn were also found in oysters (Crassostrea rizophorae) as well as of Ni and Cr in the seagrass (Ruppia maritima) and in mussels (Brachidontes exustus). Bioconcentration factors showed that the sediments are an important source for these contaminations. In such shallow and windy environment, the role of sediment resuspension linked to wind or to boat traffic must be of major importance in the dynamics of these contaminants and their bioavailability and transfer to the food-web. The role of planktonic organisms in transferring the contaminants from the water or sediment particles to the food web must be elucidated. These results highlight the necessity to maintain a monitoring of metals in the lagoon.

Faecal contamination

25Water quality of the lagoon may be temporarily and/or spatially affected by faecal pollution as shown by Soto-Castor and Esquivel-Herrera (this issue). Indeed, faecal coliform densities surpass the permissible levels and the presence of the enteropathogenic genera Salmonella and Shigella poses a further alert.

26This pollution is mainly due to the presence of human settlements and related activities as agriculture, cattle and poultry husbandry, but wildlife waterfowl and mammals also play a role. The time and space variation of abundance and distribution of faecal coliform and enteropathogenic bacteria are correlated to runoff from the surrounding human settlements. However they have no link with total heterotrophic bacteria which in turn are positively correlated to phytoplankton biomass (chlorophyll a). Spatially, the north and eastern zones differed from the south and western zones. Seasonally, total coliforms were the most abundant during the full rainy season in relation with maximal runoff influence over the lagoon. Faecal coliforms were prominent during the dry and north wind seasons particularly in the northern part in relation with the upstream sources at La Palma which also contributed to increase inorganic nitrogen and phosphorus (Esquivel & Soto, this volume). At the opposite total heterotrophic bacteria had their highest concentration in the early rainy season, benefiting from phytoplankton bloom and better oxygenation.

27The presence of the enterobacteria genera Enterobacter, Klebsiella, Escherichia, Serratia, Citrobacter, and Proteus in all surveys is linked to their adaptability to survive in conditions that are not the optimal for their growth. They were particularly abundant near the Basura stream and settlement areas (Chancarral and El Real). Escherichia and Shigella were important in all the seasons whereas Salmonella was mainly detected in the rainy season.

28In definitive the presence and punctual abundance of coliforms and pathogenic enterobacteriaceae linked to human-related activities may be really problematic particularly in the more stagnant water areas of the lagoon. However the flushing of the lagoon by tidal currents represents a defence against this problem, as underlined by Soto-Castor and Esquivel-Herrera (this issue) who note that total bacteria concentration is lower than would be expected for a tropical coastal lagoon in relation with this flushing.

Harmful algae

29Another important issue concerning the water quality of the lagoon and sanitary aspects is the presence of potential harmful phytoplankton species. Figueroa-Torres et al. (this issue) identified 46 species that are potentially harmful or toxic among the 357 phytoplankton species they inventoried by combining historical data and current data (own survey in June 2015 and February 2016). These 46 potentially harmful species are mainly represented by Dinoflagellates (20 species) and Diatoms (19 species). Among them, 12 species are toxic and five may affect the humans: the diatoms Pseudo-nitzschia pungens, Pseudo-nitzschia pungens var atlántica and Pseudo-nitzschia seriata, which produce domoic acid that causes amnesic poisoning by consumption of contaminated shellfish, the dino flagellate Dinophysis caudata that causes diarrhea and the Cyanoprokariota Lyngbya majuscula that produces dermatitis and respiratory diseases. Five other species (the dinoflagellates Gonyaulax spinifera, Phalacroma rotundatum, Prorocentrum gracile, Prorocentrum micans and Tripos furca) may cause disease and mortality among fishes and invertebrates. Most of the toxic species are present in the dry season, so special attention should be paid to their presence at this season.

30Despite toxic blooms were never detected, it appears necessary to implement a monitoring of toxic phytoplankton in the lagoon. In a context of increasing eutrophication of anthropic origin in the last years blooms of toxic phytoplankton may affect the economic production, human health and marine biota

5. Some perspectives

31The implementation of an operational hydrodynamic circulation model will help to better describe the water circulation in relation to tidal and climatic forcings and to better understand the variations of environmental conditions and living communities of the lagoon. It will also allow anticipating exceptional climate episodes (flood, drought, crisis of Anoxia...) or pollution events (eg trade ship stranding at the vicinity of the lagoon) in order to limit their impacts on the ecosystem.

32The conceptual model we present here (Figure 1) may serve as a basis for the implementation of a numerical ecosystem model that could be coupled to the hydrodynamic model in order to better understand the ecological functioning of the lagoon and to simulate its evolution in relation with different natural and/or anthropogenic forcings.

33Further investigation based on field observation and experimentation is however necessary to develop and test these models of ecosystem.

34They will also help, in complement to the data presented in this issue, to identify biological indicators and tools for ecological management and governance of fisheries through simulation of temporal evolution scenarios needed to anticipate problems and to propose concrete solutions. This issue also point out the necessity to follow or implement monitoring actions focusing sensible issues for the ecosystem (e.g. mangrove area, metal and fecal contamination of water and sediment, toxic phytoplankton, etc.).