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Ecology of the Sontecomapan Lagoon, Veracruz

 | 
Maria Elena Castellanos-Páez
, 
Alfonso Esquivel Herrera
, 
Javier Aldeco-Ramírez
, 
et al.

Part 5. Secondary producers

Biomass, size structure and trophic compartments of the metazooplankton in the Sontecomapan Lagoon (Veracruz, Mexico)

Marcela Ivonne Benítez-Díaz Mirón, María Elena Castellanos-Páez, Gabriela Garza-Mouriño, María Jesús Ferrara-Guerrero et Marc Pagano

Résumé

In this chapter, based on re-analysis of previous published data, we study the biomass, trophic component and size structure of the zooplankton in the Sontecomapan Lagoon, in order to better evaluate its trophic status and assess its possible incidence on food transfers, eutrophication and terminal productivity in this lagoon. Our results show very low values of zooplankton biomass and zooplankton/phytoplankton ratio, compared to literature data, revealing an overall low food-transfer at the basis of the food chain. The size-structure is in average dominated by small organisms which suggests a predation impact from fish on larger organisms (calanoid copepods). This impact was particularly important during the north wind season perhaps in relation with the breeding cycles of important fish species such as Cathorops aguadulce. However, tidal inputs of large-sized coastal marine organisms (e.g. Paracalanus aculeatus, Temora turbinata and Centropages velificatus) seems to compensate this “erosion” of large zooplankton in the region close to the communication channel. Our data analyses confirm the top-down effect on zooplankton, but also show that bottom-up forces affect phytoplankton (NH4 limitation) and zooplankton (phytoplankton limitation), meanwhile the top-down control by herbivorous zooplankton on phytoplankton is very low (<10 % of the phytoplankton stock day-1).

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Introduction

1Tropical coastal lagoons like Sontecomapan, are often considered as hot spots for biodiversity and are among the most productive ecosystems in the world (Mitsch & Gosselink, 1993; Day et al., 2012), but they are also very sensitive to human activities and climate variability (Cloern et al., 2016). Their high productivity combined with favorable hydrodynamic conditions, as well as the presence of sheltered areas (as mangroves) make these ecosystems good nursery areas for many fish species (Esteves et al., 2008). Fisheries of these lagoon environments are often key sectors for the economy of local population. However, the fishery potential is highly dependent on planktonic productivity that is, in itself, essential to support the diversity of higher trophic levels. The phytoplankton, first pelagic primary producer, forms the basis of the trophic network. Zooplankton is then a major link between lower and higher trophic levels hence supporting their biodiversity (Rose et al., 2010).

2Zooplankton organisms often represent essential prey for larvae, crustaceans and fish juveniles, and become a key for their recruitment (Munk, 1997; Østergaard et al., 2005). Furthermore, in lagoon and coastal marine ecosystems the zooplankton includes large proportions of early larval stages of many benthic and nektonic organisms, some having high commercial value, and are thus key for their recruitment (Archambault & Bourget, 1999). When composed mainly of herbivorous organisms, zooplankton exert a top-down control on primary production and then is a key for controlling accumulation of phytoplankton and eutrophication (Sommer & Sommer, 2006; York et al., 2014). Then, to evaluate the fishery potential of lagoon ecosystems, it is essential to evaluate the biomass and size structure of the different trophic compartments of the planktonic ecosystem (Jeppesen et al., 2000).

3Fishing represents a major consumptive heritage use of the coastal lagoons along the Gulf coast of Mexico which produce around 90,000 tons annually (Smardon, 2006). In the Sontecomapan Lagoon, fishing activity takes place both at sea and the lagoon with a 289 boat fleet. It targets mainly three species of fish (Mugil cephalus, Arius melanopus, Cathorops aguadulce, Eugerres sp.), crabs (Callinectes sp., Mennipe mercenari), and prawn (Litopenaeus setiferus). The ichthyofauna of the lagoon is very rich, with 52 fish species that belong to 24 families and 41 genera reported by Rodríguez-Varela et al. (2010), however, Castillo-Rivera & Lara-Domínguez (this issue) more recently reported 115 species corresponding to 26 orders, 41 families and 84 genera.

4In a previous work (Benítez-Díaz Mirón et al., 2014) we showed that transparency, salinity, and food availability were important drivers for zooplankton abundance, composition and diversity. We also discussed that the low zooplankton density found in the lagoon, compared to other eutrophic coastal lagoons, could be attributed to the combined effects of high water exchanges, low depth and high transparency, favoring instability and vulnerability to UV-effects and/or to visual predation.

5In this chapter, focused on the biomass and size structure of zooplankton, we completed this analysis, in order to better evaluate the trophic status of the planktonic ecosystems and assess the possible incidence on food transfers, eutrophication and terminal productivity.

Materials and Methods

6Three sampling surveys, covering a 10-station network with different characteristics (Figure 1) were performed in March (26 to 29), June (11 to 14) and November (19 to 22), 2010 corresponding to dry, rainy and north wind seasons respectively. During each survey the ten stations were sampled once for environmental variables, bacterial abundance, chlorophyll concentration and zooplankton. All sampling and measurements were performed during daylight hours (between 9: 00 and 16: 00 h). The sampling strategy, and the measurements techniques for the abiotic (transparency, pH, salinity, temperature, nutrients) and biotic (bacteria biomass and chlorophyll pigments), variables are detailed in Benítez-Díaz Mirón et al. (2014).

7The zooplankton was collected using a cylinder-conical net (64 µm in mesh opening size, 30 cm in mouth diameter and 1 m in length) and the samples were preserved with 4 % formalin.

8Zooplankton taxa identification and enumeration were performed as in Benítez-Díaz Mirón et al. (2014).

Figure 1. Sontecomapan Lagoon with the location and number of each sampling station.

Figure 1. Sontecomapan Lagoon with the location and number of each sampling station.

Individual size and weight of zooplankton organisms and estimation of zooplankton biomass

9The biovolume (µm3) of rotifers was determined by measuring 20 individuals of each species, using the formulae described by Ruttner-Kolisko (1977). The biomass was then estimated assuming a relative density of 1.0. For species that appeared less frequently, such that the total of necessary individuals for measurements were not attained, the corresponding values of biomass were obtained from Dumont et al. (1975), Sarma et al. (2008) and Basinska et al. (2010).

10The individual sizes of the other zooplankton taxa were computed from literature: summarized data for copepod species in Razouls et al. (2005-2017), mean size values of the other taxa, from Pagano & Saint-Jean (1994; lagoon Ebrié, Ivory Coast). Then the individual weight (as carbon, dry weight and wet weight units) were computed using length-weight relationships from the literature: Mauchline (1998) for calanoid copepods, Chisholm & Roff (1990) for other copepods, and Davis & Wiebe (1985) for other zooplankton taxa.

11The individual size, biovolume and weight for each taxon are summarized in Table 1.

12To assess the spectrum size, we expressed the size of all organisms with an equivalent spherical diameter computed from the wet weight values and assuming a relative density of 1.

13The biomass of each taxon (expressed as mg C m-3) was obtained multiplying the abundance values from Benítez-Díaz Mirón et al. (2014) by the individual carbon weights.

Zooplankton taxa diets and constitution of trophic categories

14Using common data from literature the zooplankton taxa were grouped into three categories according to their feeding modes (Table 1):

  • Detritus - feeders included Poecilostomatoid copepods (genera Oncaea and Corycaeus), harpacticoid copepods (genera Canuella, Tisbe, and undetermined), polychaete larvae and chironomid larvae.

  • Predators / carnivorous included cyclopoid copepods from freshwater origin belonging to the genera Mesocyclops and Thermocyclops, chaetognaths, hydrachnids, and other water mites.

  • Herbivorous-omnivorous (suspension feeders) included all other zooplankton taxa.

Estimation of zooplankton carbon demand and grazing impact

15Zooplankton carbon demand (ZCD) of suspension feeding zooplankton was computed based on estimates of biomass and ration:

16ZCD (mgC m-3 d-1) = Ration Bzoo

17where Bzoo is the biomass of zooplankton in mgC m-3, and Ration is the amount of food consumed per unit of biomass, calculated as:

18Ration (d-1) = (gz + r) / A

19where gz is the growth rate, r is the weight specific respiration and A is assimilation efficiency; gz was calculated following Zhou et al. (2010)

Table 1. Mean values of individual size (as equivalent spherical diameter, ESD, and total length), volume and weight (as wet weight, dry weight and carbon weight) of the zooplankton taxa recorded. The second column indicates the trophic group (based on literature knowledge): H=herbivorous / omnivorous, D=detritivorous, C=carnivorous

Table 1. Mean values of individual size (as equivalent spherical diameter, ESD, and total length), volume and weight (as wet weight, dry weight and carbon weight) of the zooplankton taxa recorded. The second column indicates the trophic group (based on literature knowledge): H=herbivorous / omnivorous, D=detritivorous, C=carnivorous

20as a function of sea water temperature (T, °C), food availability (Ca, mgC m-3, estimated from Chl-a), and individuals weight (w, mgC).

21Following Nival et al. (1975) we considered a constant value of A = 0.7 d-1

22Respiration, r, was estimated from gz using an empirical relationship established for tropical lagoon zooplankton by Pagano & Saint-Jean (1994):

23Ln r = 0.134 ln gz - 0.340

24We compared ZCD to the phytoplankton stock, converted to carbon assuming a classical C: Chl-a ratio of 50: 1 to estimate the potential clearance of phytoplankton by copepods.

Data analysis

25In the results, all mean values are presented with indication of standard deviation (mean ± SD).

26Two-way ANOVAs were performed to test the effects of sampling survey (dry season, rainy season and north wind season), stations on the biotic and abiotic parameters and on zooplankton. We used linear model after checking for normal distribution of each variable using Saphiro-Wilk test. Tukey’s post-hoc test of honest significant difference (HSD) was also performed to compare the mean group values.

27Stepwise multiple regression analyses were conducted to explain the variability in zooplankton and phytoplankton biomasses. Relationships were tested between these biomasses and variables describing main environmental conditions (temperature, salinity, oxygen), food resources (nutrients for phytoplankton and phytoplankton for zooplankton) and potential consumers (herbivorous zooplankton for phytoplankton, and fish and decapod larvae for zooplankton).

28The spatial and seasonal variability of biomass percentages of zooplankton groups or biomass percentages of zooplankton size classes were tested using principal component analyses (PCA). In each analysis zooplankton and phytoplankton biomass, temperature and salinity were considered as additional variables. To increase homoscedasticity, all the data were transformed (log + 1) before analysis.

29ANOVA and regression analyses were performed with Statistica v6 software. PCAs were performed with PRIMER 6 software.

Results

Taxonomic and trophic composition of the zooplankton biomass

30Zooplankton biomass varied between 0.1 and 230 mgC m-3 with a very high outlier value (230 mg m-3) at station 4 during the dry season linked to a zooplankton patch (mostly due to the cyclopoid copepod Oithona nana) (Fig. 2), thus this value was discarded from the statistical data analysis. The mean biomass was significantly higher during the dry season than during the rainy and north wind seasons (ANOVA, p=0.018; Fig. 2) whereas no significant difference was found between stations despite a very high mean value at station 6 (Cocal) (Fig. 2) due to the maximal value (35 mg C m-3) recorded during the dry season. The zooplankton biomass was strongly dominated by copepods except in some occasions where other holoplankton (station 1, dry season, stations 7 and 9, rainy season) or meroplankton (stations 2 and 3, dry season, stations 2 and 5, north wind season) became very abundant and even dominant. Cladocerans and rotifers represented a very low fraction of the biomass, and almost disappeared during the north wind season together with other holoplankton, therefore, during the north wind season the zooplankton biomass was quite exclusively composed of copepods and meroplankton organisms.

31The comparison of phytoplankton biomass with the biomass of the main zooplankton trophic groups revealed a very unbalanced trophic chain, with a very strong biomass decrease between the first two trophic levels (phytoplankton and herbivorous); in most cases, except at station 4 during the dry season, due to the patch of Oithona nana (Fig. 3). The mean herbivorous/phytoplankton ratio was 1.5 % ± 0.28 % with most values <1 %. Detritivores represented up to 45 % of the zooplankton biomass, with a mean percentage of 7.9 ± 2.2 %. Predaceous zooplankton was always <1 % of the biomass and were absent in many cases.

Biomass size-structure

32The zooplankton biomass was dominated by small organisms of the size-class <200 µm, representing up to 97 % of the total biomass (mean = 33 ± 5 %; Fig. 4). The largest size-class (>600 µm) was the second in terms of importance (0 to 95 % of the biomass; mean = 19.9 ± 5.2 %), whereas the intermediate class (300-400 µm) was the less representative (0-13 %; mean = 1.9 ± 0.6 %). This resulted roughly in a bimodal size-structure with two peaks, for small and large organisms. This bimodal size-distribution disappeared during the north wind season, with a significant increase of the smallest class (<200µm) and a significant decrease of the largest one (>600 µm), as compared to the two other seasons (ANOVA, p=0.008 and 0.016 respectively). In stations close to the communication with the ocean (stations 7, 8 and 9) the size-distribution was shifted towards large organisms with a significant reduction of the smallest class (<200 µm) and a significant increase of the largest one (>600 µm) as compared with the other stations (ANOVA, p=0.003 and 0.045 respectively).

Figure 2. Variations for total zooplankton biomass and for biomass percentages of the main zooplankton taxonomic groups during the dry (A), rainy (B) and north wind (C) seasons.

Figure 2. Variations for total zooplankton biomass and for biomass percentages of the main zooplankton taxonomic groups during the dry (A), rainy (B) and north wind (C) seasons.

Figure 3. Variations for phytoplankton biomass and for the main zooplankton trophic groups during the dry (A), rainy (B) and north wind (C) seasons.

Figure 3. Variations for phytoplankton biomass and for the main zooplankton trophic groups during the dry (A), rainy (B) and north wind (C) seasons.

Figure 4. Variation for biomass percentage of the six zooplankton size-classes: mean values per season (A) and sampling station (B).

Figure 4. Variation for biomass percentage of the six zooplankton size-classes: mean values per season (A) and sampling station (B).

33If we exclude the abnormally very high value at station 4 during the dry season, the zooplankton grazing pressure varied between 0.1 and 46 µgC l-1 d-1 (mean = 4.9 ± 1.6 µgC l-1 d-1) which represented 0.1 to 9.8 % of the phytoplankton stock removed daily (mean = 2.0 ± 0.4 % d-1; Fig. 5). The grazing pressure was significantly higher during the dry season than during the north wind season (ANOVA, p=0.042) but did not differ significantly between sampling stations. It was significantly correlated to the phytoplankton biomass (r=0.585; p=0.001) suggesting that bottom-up control prevailed.

Figure 5. Variations for zooplankton grazing and % grazing impact during the dry (A), rainy (B) and north wind (C) seasons.

Figure 5. Variations for zooplankton grazing and % grazing impact during the dry (A), rainy (B) and north wind (C) seasons.

Multiple regression analysis for Phytoplankton and Zooplankton biomass

Table 2. Equation parameters of the multiple linear regression models, significances and partial correlation coefficients

Table 2. Equation parameters of the multiple linear regression models, significances and partial correlation coefficients

34The multiple regression model for zooplankton (R2 = 0.48; p<0.0072) shows that the variability of the zooplankton biomass is positively related to phytoplankton and negatively related to fish larvae whereas temperature and salinity do not enter significantly into the model. This suggests coupled bottom-up (phytoplankton) and top-down (with fish-larvae as predator index) controls of the zooplankton biomass.

35The multiple regression model for phytoplankton (R2 = 0.49; p<0.028) suggests bottom-up control of phytoplankton biomass with NH4+ contributing significantly (p=0.018), whereas top-down control (with herbivorous biomass as a proxy) has no significant effect (Table 2).

Principal component analyses (PCA)

36The first factorial plane of PCA on biomass percentages of zooplankton taxonomic and trophic groups explained 65 % of the variance of which 37.5 % were attributed to the first axis and 27.5 % to the second (Fig. 6). The first axis opposes most sampling points of the dry season, characterized by high zooplankton and phytoplankton biomasses and by high percentage of non-copepod holoplankton (others), to the sampling points of the north wind season, characterized by lower plankton biomass and importance of meroplankton and detritivorous organisms.

Figure 6. PCA on biomass percentages of zooplankton groups and trophic categories with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.

Figure 6. PCA on biomass percentages of zooplankton groups and trophic categories with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.

37The first factorial plane of PCA on biomass percentages of zooplankton size classes explained 63 % of the variance of which 40.1 % were attributed to the first axis and 23.0% to the second (Fig. 7). The first axis opposes almost all sampling points of the north wind season, characterized by abundance of smaller size-classes (<300 µm), to sampling points of the two other surveys (Dry and Rainy), characterized by larger sizes and higher total biomass. The second axis mainly shows spatial opposition during the north wind season between stations close to the ocean (stations 7, 8 and 9) characterized by larger size classes (500-600 µm) and stations of the central part of the lagoon (stations 2, 3, 4, 5 and 6) characterized by small organisms (100-200 µm).

Discussion

Low zooplankton biomass in Sontecomapan compared to other coastal lagoons

38Simultaneous data on phyto- and zooplankton biomass of coastal lagoons are rather scarce, thus it is difficult to compare our data to that from other sites of the world. However, the zooplankton biomass in Sontecomapan (0.1 to 34 mg C m-3; mean = 3.5 mg C m-3) appears very low compared to literature data. From several studies we could summarize in Table 3, Sontecomapan is the poorest in terms of zooplankton biomass despite the fact that its phytoplankton level is in the range of the other studies. Zooplankton biomass reported in this study is one magnitude lower than the second poorest lagoon in the list (Ebrié lagoon, Ivory Coast; Pagano & Saint Jean, 1994) and more than 100 times lower than those reported in NW Mediterranean lagoons (Lam Hoai & Rougier, 2001; Badosa et al., 2007). Also, it is far poorer than the other Mexican coastal lagoons for which zooplankton biomass were reported, either from the Atlantic coast (Alvarado lagoon; Cruz-Escalona et al., 2007) or the Pacific coast (Huizache-Caimanero Lagoon; Zetina-Rejón et al., 2003).

Figure 7. PCA on biomass percentages of zooplankton size classes with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.

Figure 7. PCA on biomass percentages of zooplankton size classes with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.

39Another characteristic of Sontecomapan is the very low zooplankton/phytoplankton biomass ratio (1.4 %), the lowest reported in Table 3. This low ratio may arise from (i) poor (or inefficient) transfer between the first trophic levels in the pelagos and/or (ii) strong predation pressure by zooplanktophagous organisms. The second hypothesis seems realistic due to high water transparency, favoring visual predation for example from juvenile fish forms (Benítez-Díaz Mirón et al., 2014). Besides, this low biomass ratio is comparable to the one reported by Pagano & Saint Jean (1994) in another Atlantic tropical lagoon (Ebrié lagoon; W Africa) where it was shown that the low zooplankton biomass compared to phytoplankton was mostly due to strong predation pressure by an important mysid (Rhopalophthalmus africana) population (Kouassi et al., 2006).

Zooplankton biomass size-structure in Sontecomapan suggests top-down control by fish

40The size spectrum is considered as a key indicator of the zooplankton assemblages dynamics (Krupica et al., 2012). It also reflects the status of the zooplankton community in relation with trophic bottom-up and top-down forcing (Hall et al., 1976; Pace, 1986), and has been shown to be a good indicator of the fish community structure (Mills et al., 1987). Any changes in zooplankton size-structure, may thus reflect changes in the ecosystem structure and functioning in relation with environmental context modification, driven by climatic change and/or anthropogenic pressure (Brucet et al., 2010). In Sontecomapan, the biomass size-structure is, in average, dominated by small organisms in the size-class of <200 µm (up to 97 % of the total biomass, with mean of 33 ± 5 %), including mostly rotifer species. This dominance of small organisms may reflect strong predation pressure from larvae or juvenile fish on larger zooplankton organisms, particularly in the 300-600 µm size range, which includes calanoids like Acartia tonsa, and harpacticoid copepods like Euterpina acutifrons and Tisbe spp. (Table 1). Such top-down control by fish on Sontecomapan zooplankton is also suggested by the results from the multiple linear-regression analysis showing a significant negative relationship between zooplankton biomass and fish larvae. The mullet Mugil cephalus well represented in the lagoon may exert a strong predation pressure on these copepods as its larvae eat zooplankton almost exclusively (Eda et al., 1990) and its younger stages are known to prey on copepods near the sediment during the day (Eggold & Motta, 1992), when copepods migrate close to the bottom. Besides, mugilids have been shown to play a key role in shallow lakes by removing sediment (and indirectly nutrients) and large zooplankton, most likely influencing the dominance of smaller forms like rotifers (Oltra et al., 2001; Blanco et al., 2003). Catfish Cathorops aguadulce, well represented in Sontecomapan lagoon as well as in other coastal lagoon of the Gulf of Mexico, was also shown to spawn in different habitats of the lagoons using the seagrass/mangrove system as a nursery area. This suggests an important larval density of Cathorops aguadulce in Sontecomapan, probably exerting strong predation on zooplankton. High predation may be also exerted by their juvenile and adult stages which showed great percentages of copepods in their diet (87 % and 47 % of total prey numbers respectively) at Téminos Lagoon, Mexico (Yáñez-Arancibia & Lara-Domínguez, 1988).

41In this study, we observed spatial and temporal changes on the zooplankton size-structure in Sontecomapan. In terms of seasonal variations, the dominance of small organisms was more accentuated during the north wind season mainly due to depletion of the largest size-class (>600 µm) compared to the other seasons (Fig. 4). This seasonal variation in zooplankton size-structure may be linked to fish breeding cycles, leading to seasonal variation of the predation pressure from fish larvae on large calanoid copepods such as Paracalanus aculeatus, Temora turbinata and Centropages velificatus, which make the bulk of the largest zooplankton size-class (Table 1). For example, it was shown in Términos Lagoon (Mexico) that Arius melanopus carried seasonal migrations for reproduction and feeding with a peak spawning period just before the north wind season (Yáñez-Arancibia & Lara-Domínguez, 1988). Spatially, the dominance of small organisms was permanent in the central and southern part of the lagoon, but disappeared in the channel communicating with the ocean (stations 7, 8 and 9), where the size-distribution was shifted towards large organisms with a significant increase of the largest size-class (>600µm) as compared with the other stations. This may correspond to variations of zooplankton composition linked to marine water influence, with rather stable salinity conditions in the central and southern part of the lagoon contrasting with tidal influence in the northern part driving inputs of marine coastal species as Paracalanus aculeatus, Temora turbinata and Centropages velificatus in the channel and rotifers collapse due to high salinity pulses.

Trophic controls of phyto- and zooplanktons

42If a top-down control on zooplankton by fish predation is suggested by biomass ratio analysis and the size-structure (see discussion above), our regression analysis also shows clearly that the zooplankton biomass is positively related to phytoplankton suggesting a bottom-up control linked to food (phytoplankton) limitation. Such food limitation, despite the rather high chlorophyll level (Table 3) may result from inedibility of a large part of phytoplankton. In Sontecomapan during the study period, high concentration of chlorophyll c with importance of c1 and c2 forms (Benítez-Díaz Mirón et al., 2014) suggested large proportion of dinoflagellates and diatoms, from which several species may be toxic to zooplankton (Granéli & Turner, 2006), also, blooms of diatoms (Skeletonema spp., Cyclotella spp. and Chaetoceros holasticus) and dinoflagellates (Peridinium spp., Prorocentrum cordatum) have been reported (Aké-Castillo & Vázquez, 2008).

43The multiple regression model for phytoplankton suggests a bottom-up control of phytoplankton biomass linked to nitrogen limitation (NH4+) as also suggested by the overall low N/P nutrient ratio in the lagoon (6.4±1.5), much lower than the Redfield ratio. Aké-Castillo & Vázquez (2008) also pointed out the importance of bottom-up effects on phytoplankton in Sontecomapan with clear seasonal and spatial variations linked to organic matter mineralization introduced by mangrove litter.

Table 3. Mean and range values comparison of phytoplankton and zooplankton biomass with literature data on other coastal lagoons

Table 3. Mean and range values comparison of phytoplankton and zooplankton biomass with literature data on other coastal lagoons

References: 1 = Lam Hoai & Rougier (2001), 2 = Badosa et al. (2007), 3 = Casagranda & Boudouresque (2010), 4 = Pagano & Saint (1994a)

44On the other hand, top-down control by herbivorous organisms on phytoplankton would be non-significant as shown by the regression analysis. Besides, our grazing estimates show that the grazing pressure from herbivorous zooplankton on phytoplankton is rather low, representing only 0.1 to 9.8 % of the phytoplankton stock removed daily.

Conclusion

45In Sontecomapan, the low zooplankton abundance is related to strong predation by larval and juvenile fish, while the food selectivity of herbivorous zooplankton would lead to a low exploitation of the phytoplankton production, and thus to reduced zooplankton stocks. This should foster the sedimentation of ungrazed dead cells and the accumulation of organic matter at the bottom that could lead to the lagoon’s silting up and anoxia events. This tendency 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 with priority.

Acknowledgements

46The authors are thankful for the financial support contributed by the France-Mexico ECOS-ANUIES 2010-2014 cooperation program. MIBDM is thankful for the scholarship granted by CONACyT (227103/46776), for obtaining the degree of Doctor in Biological and Health Sciences.

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Table des illustrations

Titre Figure 1. Sontecomapan Lagoon with the location and number of each sampling station.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-1.jpg
Fichier image/jpeg, 33k
Titre Table 1. Mean values of individual size (as equivalent spherical diameter, ESD, and total length), volume and weight (as wet weight, dry weight and carbon weight) of the zooplankton taxa recorded. The second column indicates the trophic group (based on literature knowledge): H=herbivorous / omnivorous, D=detritivorous, C=carnivorous
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-2.jpg
Fichier image/jpeg, 182k
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-3.jpg
Fichier image/jpeg, 4,2k
Titre Figure 2. Variations for total zooplankton biomass and for biomass percentages of the main zooplankton taxonomic groups during the dry (A), rainy (B) and north wind (C) seasons.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-4.jpg
Fichier image/jpeg, 49k
Titre Figure 3. Variations for phytoplankton biomass and for the main zooplankton trophic groups during the dry (A), rainy (B) and north wind (C) seasons.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-5.jpg
Fichier image/jpeg, 67k
Titre Figure 4. Variation for biomass percentage of the six zooplankton size-classes: mean values per season (A) and sampling station (B).
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-6.jpg
Fichier image/jpeg, 52k
Titre Figure 5. Variations for zooplankton grazing and % grazing impact during the dry (A), rainy (B) and north wind (C) seasons.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-7.jpg
Fichier image/jpeg, 35k
Titre Table 2. Equation parameters of the multiple linear regression models, significances and partial correlation coefficients
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-8.jpg
Fichier image/jpeg, 50k
Titre Figure 6. PCA on biomass percentages of zooplankton groups and trophic categories with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-9.jpg
Fichier image/jpeg, 12k
Titre Figure 7. PCA on biomass percentages of zooplankton size classes with zooplankton and phytoplankton biomass, temperature and salinity as additional variables: plot of vector variables and stations with color-code according to seasons.
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-10.jpg
Fichier image/jpeg, 13k
Titre Table 3. Mean and range values comparison of phytoplankton and zooplankton biomass with literature data on other coastal lagoons
Légende References: 1 = Lam Hoai & Rougier (2001), 2 = Badosa et al. (2007), 3 = Casagranda & Boudouresque (2010), 4 = Pagano & Saint (1994a)
URL http://books.openedition.org/irdeditions/docannexe/image/35509/img-11.jpg
Fichier image/jpeg, 48k

Auteurs

Universidad Autónoma Metropolitana, Unidad Xochimilco. Departamento El Hombre y su Ambiente. Laboratorio de Rotiferología y Biología Molecular de Plancton. Calzada del Hueso 1100, Col. Villa Quietud, Coyoacán, C.P. 04960, CDMX. Marcela Ivonne Benítez-Díaz Mirón, . Phone number: +52 (0155) 54837181.

Universidad Autónoma Metropolitana, Unidad Xochimilco. Departamento El Hombre y su Ambiente. Laboratorio de Rotiferología y Biología Molecular de Plancton. Calzada del Hueso 1100, Col. Villa Quietud, Coyoacán, C.P. 04960, CDMX. Marcela Ivonne Benítez-Díaz Mirón, . Phone number: +52 (0155) 54837181.

Universidad Autónoma Metropolitana, Unidad Xochimilco. Departamento El Hombre y su Ambiente. Laboratorio de Rotiferología y Biología Molecular de Plancton. Calzada del Hueso 1100, Col. Villa Quietud, Coyoacán, C.P. 04960, CDMX. Marcela Ivonne Benítez-Díaz Mirón, . Phone number: +52 (0155) 54837181.

Universidad Autónoma Metropolitana, Unidad Xochimilco. Departamento El Hombre y su Ambiente. Laboratorio de Ecología Microbiana. Calzada del Hueso 1100, Col. Villa Quietud, Coyoacán, C. P. 04960, CDMX.

Aix-Marseille Univ., Mediterranean Institute of Oceanography (MIO), 13288 Marseille cedex 09, France; Université du Sud Toulon-Var, 83957 La Garde cedex, France; CNRS/INSU, MIO, UMR 7294, 13288 Marseille cedex 09; IRD, MIO, UR235, 13288 Marseille, France.

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