Coastal ecosystems are highly productive, vulnerable, and particularly for tropical coasts, are greatly diverse with respect to both species and habitats. Displayed food webs highly complex, characterized for the extraordinary interchange of species from ecosystems interconnected. A noteworthy progress is the being to have recognized that both the structure and function of the trophic network are dynamic properties of the system. Food web describes species interaction and is an important part of community structure [1-5].

Currently, the assessment of what are the processes and mechanisms that determine the natural communities structure, it is an area under intense research [6-18]. Although the processes involved in regulation of communities such as predation, competition, mutualism, parasitism, etc., are long known in the field of ecology, the relative incidence of these processes in the community structure regulation, remains as hypothesis in most cases, especially in natural communities.

The network analysis has established that the natural communities’ structure is not a random result [6, 9, 19, 20], but is the result of the precise combination of several processes. Many of these regulatory processes are emergent from the interaction between three or more species (high order interactions, indirect effects, interactions modifications), so it cannot be observed or inferred by studying the interaction between species pairs [9, 21-25]. In this context, studies based on trophic interactions are the first step in the development of hypotheses about the homeostatic feedback of natural communities, consequently; the comparative analysis of the diet composition, allows building food webs schemes that serves as a basis for proposing diverse hypotheses on the regulation and dynamics of the system under study.

In other hand, the study of the mechanisms that determinate the structure and dynamics of marine communities under fisheries exploitation is particularly relevant because the importance of the adequate management of exploited populations, even so, it is common the extensive use of analytical approaches [26-30].

Furthermore, the large-scale fisheries emergent around the world over the last thirty years has caused a shift in how to approach the management, resulting in the gradual incorporation into the handling of variables not directly related to the stocks under exploitation (e.g. the influence of other species that can modify the abundance of commercial species by interespecific relationships as predation or competition); that means the incorporation of those variables into dynamic models of fish stocks [31].

The understanding of food web changes is one of the major issues of modern ecology [32-37]. By analyzing how groups are assembled and develop, would give an insight into the organization of biological ecosystems. In particular, one of the main focuses of food web theory is to understand how structural properties change with the scale of the system [38, 39]. Different methodological approaches have been proposed to quantify the magnitude of the relationships between species or species groups, and to assess the relative importance of each component has on the overall system maintenance [40], The most common approach employed in the last four decades, biomass balance, Ecopath models [41, 42]. Habitat damage and mishandling of resources could increase mortality along trophic web [43-46], and modify the relationship between functional groups [47, 48].

Particular attention has been paid to the development of ecosystem models (mass-balance, Ecopath) that synthesize the trophic interaction patterns of a particular food web. These models have been widely accepted by the international scientific community, having been applied to more than 150 ecosystems around the world [41, 42]. Ecopath is a theoretical approach that encourages the development of trophic models of aquatic ecosystems through mass- adjusting; also, permits examination of various aspects of the subsequent food web network. Input data required includes estimates of biomass, production, consumption, diet and harvests for each group considered [49].

The study of ecological networks has centered around their inward structural and / or functional characteristics (e.g. biological process, inter or intra relationships, food connections). A noteworthy progress of those lines of investigation has been the acknowledgement that food webs are closely related to the whole system´s dynamic. However, few efforts have been made to study the structure and association of interconnected marine waterfront biological communities, which the exchange of energy (matter) through trophic flows is recognized.

On the other hand, commercial fishing catches have decreased substantially in recent years in south-central Gulf of Mexico [50]. As a result, human impacts on the local environment are of increasing concern [51, 52]. In this sense, Ecopath models represent a modelling approach alternative to evaluate changes in the coastal marine ecosystems being able to be structural or functional. Direct and indirect effects of species on others into the system can also be explored, as well as the overall functioning of the ecosystem. However, it is important to note that our knowledge on direct and indirect effects of species, is limited and it is therefore necessary to develop a greater number of models to find ecosystem attributes that can be used as biological reference points, similar to those used in models conventionally used in fisheries biology [53].

The main activity in Mexican waters is the shrimps trawl fishery, whit a large bycatch most of the times consisting of juveniles of commercially important teleostean fishes such as croakers, pompanos, snappers, groupers, etc., most of which is discarded. In last years, fisheries production has declined [50] besides, some biological parameters as the average size of individual fish [54]. Also, data presented by Abarca-Arenas et al. [55, 56] on bycatch of shrimp fishery boats trawlers and observations by Cházaro-Olvera et al. [51] suggest a marked increase in the abundance of by-catch and portunid crabs in the shrimp by-catch in the Gulf of Mexico.

In this study we develop a trophic model to characterize the structure and function of two coastal marine ecosystems, Alvarado lagoon (Mexico) and the adjacent continental shelf, both considered important areas for penaeid shrimps and demersal fish. These ecosystems are characterized to interchange biota and, therefore, matter and energy through a feeding relationship, since they are connected through an artificial mouth in the northern at Camaronera lagoon and by a natural mouth in the southern part of the system. In order to address this objective, we examined 66 functional groups based on estimates of biomass, production, consumption, diet and harvest for each group, using Ecopath model of biomass balance that aid to synthesize the trophic interaction patterns of a particular ecosystem. This approach ensures a better description of the trophic relationships, energy flows, and transfer efficiency of the food web.

The Ecopath model presented here synthesizes the biological and ecological information available for coastal marine ecosystems coupled: Alvarado lagoon, Mexico and continental shelf adjacent. Several of the species included in the model use both systems to complete some of the stages in their life cycles, whether for reproductive purposes, breeding, protection or feeding [86-88] the exchange of biomass of these species between one system and another is clear, evidencing the need to make a nested model like the one presented here in a first approach. The model used here offers important information regarding the ecosystem’s structure, function, and energy flows, providing a means for comparison with other ecosystems in terms of the system’s energy base and maturity indicators, following Odum [75, 89, 90].

Based on the results of this study, detritophages and non-primary producers are responsible for the transfer of energy toward higher levels. However, the ecotrophic efficiency of detritus was relatively low (EE detritus shelf = 0.42, EE detritus lagoon = 0.47). This could be interpreted as the excessive production of detritus such that only a small proportion is consumed within the system and therefore, exportation and accumulation may occur.

A similar pattern has been observed for upwelling systems, where primary producers tend to have low ecotrophic efficiency ratings [91-93]. They produce such large quantities that little of this biomass is effectively used by the other trophic levels.

Most functional groups making up the trophic structure of the system occupied intermediate trophic levels (II-III), a situation common previously reported for other continental shelf ecosystems [94-96]. This pattern can be attributed to a strong reliance on primary producers [95, 97].

All ecosystems, whether aquatic or terrestrial, generally include four basic functional groups (autotrophs, primary consumers, secondary consumers, and decomposers). Energy and materials flow from autotrophs to degraders via a web of biotic interactions and trophic relations. The production of biomass at each trophic level, the magnitude and velocity with which energy and materials flow through the web, and the complexity of that web vary. However, the majority of the energy and materials are concentrated in the autotroph group, decreasing as they flow towards primary consumers, secondary consumers, and degraders [98].

Some previous works have reported structures dominated by functional groups from higher trophic levels [95, 99]. Others, have reported that different organisms inhabiting coastal environments uses resources from both marine and terrestrial sources. These sources vary by location, and it could include energy from detritus and nutrients (discharged amid their disintegration or absorption) from both local and external sources (macrophytes and phytoplankton) as well as fresh water and estuarine phytoplankton, macrophytes and terrestrial sediments [100-102].

The ecotrophic efficiency of most groups was less than 0.5 (Table 1), suggesting they are not exposed to high mortality rates due to predation or high rates of exploitation due to fishing, or rather that the present exploitation rates do not affect their biotic potential. Functional groups considered species forage (small or intermediate-sized pelagic o demersal species), including Mullidae, meiobenthos, and other crustaceans, had ecotrophic efficiencies > 0.90. Excluding macrophytes (shelf) the majority of the groups with low ecotrophic efficiencies (< 0.1) were apex predators in the trophic web. The low ecotrophic efficiency of some benthic groups can be attributed to their abundance (polychaetes, B = 6.28 tk^-2year^-1; other crustaceans, B = 3.48 tk^-2year^-1; bivalves, B = 2.98 tk^-2year-1; echinoderms, B = 2.29 tk^-2year^-1) and the reduced levels of predation they are subjected. Particularly, meiobenthos play a relatively more important role in the transfer of energy, maybe result of their higher renovation rates [62]. As a result, meiobenthos metabolic requirements (secondary production and respiration) may be greater than those required by macrofauna, particularly in ecosystems where the ratio of macrofauna biomass to meiofauna biomass is less than, 5:1 [103, 104]. Thus, the importance of studies regarding the role of meiofauna has been recently acknowledged [62, 105-109], as their role in the benthic trophic web is considered analogous to that of zooplankton in pelagic systems [110].

Interestingly, zooplankton are responsible for approximately 80% of phytoplankton mortality yet they have a relatively high ecotrophic efficiency (EE = 0.77). The reason is that this group serves as forage for several components of the trophic web. Thus, zooplankton have an important role in controlling transfer of energy, facilitating the flow of the elevated production of phytoplankton toward higher trophic levels [111]. These in turn prey intensively on zooplankton (e.g., the cephalopod L. pealei, and some teleosts groups such as Clupeidae and Gerreidae).

The spine Lindeman diagram (Fig. 2) shows that the low levels quantitatively dominate flows and biomass in the system, the role of detritus as the primary source of energy, which stands out. This finding is in agreement with that reported by several other works [95, 96] for other Gulf of Mexico ecosystems. In contrast to Odum’s findings [75], the predominance of the detritus route does not appear to reflect the ecosystem´s maturity. Together, the indicators of omnivory index (SOI = 0.233), connectivity (CI = 0.125), and ascendency (A = 11,029 flowbits ≈ 26.9%), suggest that the system is a developing ecosystem.

The more intensive use of detritus may be related to: ocean circulation patterns created by cyclonic disturbances in the area (which facilitate the re-suspension and confinement of sediments and nutrients), nutrients from water discharged by the various rivers in the region (i.e., Coatzacoalcos, Papaloapan, Blanco, Acula), elevated phytoplankton production (a considerable proportion of which flows toward detritus), benthic biomass (composed of several detritovores), and the resuspension of sediments by trawl fishing [58, 62, 63, 112].

The Lindeman spine (Fig. 2) shows biomass (fluxes) that each component obtain from previous trophic level, also shows the biomass (energy) leaving by other processes as respiration or export, and the net production passed on to the next higher level. The average transfer efficiency (13%) is consistent with the proposed by Lindeman [71] and support for Christensen and Pauly [113] for several coastal marine ecosystems. Most of the output comes from the lowest trophic levels (~95%), while the remaining 5% of the flows derive from the highest trophic levels.

The TPP/TB rate was relatively high, possibly indicating that the ecosystem is in an advanced state of eutrophication (Table 3). Similar values have been observed for other systems with clear signs of environmental deterioration. For example, Barausse et al. [114] reported a TPP/TB ratio equal to 14.5 for the Adriatic Sea, while Heymans et al. [92] published a TPP/TB ratio equal to 16.2 for the northern Benguela upwelling ecosystem. Our interpretation of the TPP/TB (Table 3) for the Alvarado lagoon-continental shelf adjacent ecosystem as evidence of eutrophication is in accordance with Caso et al. [115] and Guentzel et al. [58] finding that the Gulf of Mexico displays varying levels of eutrophication. However, it is important to exercise caution when using this indicator as it is influenced by the functional groups employed to build the model.

Fig. (5). Keystoneness for the functional groups of the Alvarado lagoon and adjacent continental shelf model. For each functional group, the keystone index is reported against overall effect. Overall effects are relative to the maximum effect measured in the trophic web. The species are ordered by decreasing keystoneness. 1. Sea mammals, 2. Sea mammals, 3. Sea birds, 4. Sharks, 5. Rays/skates, 6. Rays/skates, 7. Mackerels, 8. Goatfish, 9. Flatfish, 10. Flatfish, 11. Croakers/drums, 12. Croakers, 13. Herrings, 14. Herrings, 15. Lizardfish, 16. Gruopers, 17. Jacks/pompanos, 18. Jacks, 19. Filefish, 20. Cutlassfishes, 21. Barrucadas, 22. Needlefish, 23. Searobins, 24. Snappers, 25. Snappers, 26. Mojarras, 27. Mojarras, 28. Pinfish/porgies, 29. Pinfish/porgies, 30. Sea catfish, 31. Sea catfish, 32. Snooks, 33. Snooks, 34. Mullets, 35. Mullets, 36. Grunts, 37. Morays, 38. Toadfish, 39. Toadfish, 40. Sleepers, 41. Other fishes, 42. Squids, 43. Octopuses, 44. Crabs, 45. Crabs, 46. Penaeid shrimps, 47. Penaeid shrimps, 48. Others crustaceans, 49. Echinoderms, 50. Gastropods, 51. Gastropods, 52. Polychaetes, 53. Polychaetes, 54. Bivalves, 55. Bivalves, 56. Meiobenthos, 57. Meiobenthos, 58. Zooplankton, 59. Zooplankton, 60. Phytoplankton, 61. Phytoplankton, 62. Sea grass, 63. Benthic producers, 64. Shrimp by-catch, 65. Detritus, 66. Detritus, 67. Bottom shrimp trawling, 68. Gillnets, 69. Atarrayas.

The TPP/TR rate is also used to assess damage related to human activities. In the first phases of ecosystem development, the TPP/TR should be > 1 as production exceeds respiration; in contrast, systems subjected to organic contamination should have ratios < 1, but if there is a balance between energy production and maintenance costs in mature systems, then the ratio is close to 1 [73, 89]. The SOI also is one of the highest reported for the Gulf of Mexico [94]. If we consider the SOI a measure of the feeding strategies displayed by a given system, we can infer that the trophic web of the study area is composed of functional groups (tropho-species) with relatively broad trophic niches or, alternatively, by components with a certain degree of feeding plasticity, permitting them to adapt to food resource variability. Several works have presented contrasting theories regarding whether omnivores stabilize or destabilize trophic webs; some studies, and the supporting empirical evidence, suggest that omnivores destabilize the trophic web [116-120]. Other theoretical studies including the strength of trophic interactions suggest that omnivory may have a stabilizing effect when trophic connections are not strong [121, 122]. Still other studies suggest that some trophic webs undergo structural changes as a result of invasion by exotic species [123] and exploitation by higher trophic levels [46, 124].

On the other hand, the connectance index was lower than the average value reported for other coastal ecosystems in Mexico (Table 2). This suggests that the complexity of the system trophic web is probably quite highly interconnected, considering that it only represents 12% of the maximum number of possible connections.

The mixed trophic impacts and key species indices suggest that the groups with the greatest influence on the system belong to lower trophic levels (meiofauna, detritus, phytoplankton, and zooplankton), significantly affecting groups of fish and invertebrates (trophic level II-III). However, based on the results is not possible to observe any kind of control of the food web, top-down or bottom- up. The importance of benthic diversity that has been underlined in numerous studies [62, 125]; it is clearly observed in this study, the benthos in the Alvarado Lagoon represents a major trophic resource, and plays an important role in the biogeochemical budget of such as shallow system. Finally, it is also important to mention the role of surplus phytoplankton production and detritus, which is exported to the adjacent continental shelf, through the trophic web of different shared functional groups [126-129].

Except for coastal sharks and marine mammals (shelf), predator functional groups (mainly teleostean and cephalopods species) do not seem to significantly impact their prey, suggesting that higher trophic levels do little to move biomass toward the interior of the ecosystem. Before they were fished intensively, apex predators like coastal sharks, rays/skates, groupers and snappers, were present in greater quantities in the Gulf of Mexico [130-133]. Thus, they presumably had a greater impact on the trophic web, this suggestion requires further data supporting. Results presented here suggests that the ecosystem is dominated by small, fast growing organisms that are resistant to anthropogenic effects, including meiofauna, small demersal fish (Mullidae), and some cephalopods species (Octopus spp. and L. pealei). These organisms have biological traits (high renovation rates) that allow them to recover quickly under considerable fishing pressure and moderate eutrophication, compared to longer living organisms with slower metabolisms. Moreover, these organisms are able to increase their numbers quickly in response to predation.

The ascendency system is in line with values reported for other areas of the continental shelf of the Gulf of Mexico, i.e., Campeche Sound [95]. The ascendency/ capacity ratio, considered a measure of organization and efficiency, may also be seen as a measure of ecosystem maturity and an indicator of the system’s ability to resilience to perturbation [84, 134]. The A/DC ratio is similar to the value reported for other continental shelves [94]. Lower values indicate that a given ecosystem is immature and better able to resist external perturbations [135, 136]. However, the A/C ratio should be considered with care as some authors have found it to be negatively correlated with maturity [76, 137]. Maturity indices suggest that the system is a developing and relatively stable system that can continue to resist human (primarily fishing) or natural impacts without substantial modification of its structure and function, and that relatively few ecosystem components will be greatly affected in the near future. However, over the long term, if commercial fishing takes exceed the biotic potential of species of commercial interest these, and other groups not targeted by extractive activities, may decline.

Finally, we should emphasize that our model was constructed using information available from published sources, thus, the results may change as more and better data become available, and as our methodological techniques improve. Although our study did not include dynamic simulation, we are confident that the model presented here will serve as the basis for identifying gaps in available data and highlighting new areas for investigation. Also it is important to recognize that based on our results we recommend working with models that couple (nested) subsystems, because individual functioning of each one influences the operation of the other, in terms of trophic flows in both directions (interchange of species or stages), which can have substantial implications for the sound management of resources with a ecosystem approach. Moreover, this model may be used for dynamic and spatial simulations to consider the simultaneous use of resources as well as a variety of economic practices like fishing, tourism, and crude oil extraction, restoration, etc.