Synonyms
Trophic structure; Trophic web
Definition
Trophic dynamics is the basic process of transference of energy from one trophic level to the next in an ecosystem.
Introduction
The basic process of energy transfer in ecosystems occurs through trophic dynamics (Lindeman, 1942). Trophic dynamics in tropical and subtropical estuarine and coastal marine ecosystems are characterized by a broad heterogeneity, which is determined by highly diverse biological communities and complex trophic webs with a high degree of interaction (Manickchand-Heileman et al., 1998). The dynamic nature of trophic web interactions and the variation in structure and function of several coastal systems (including estuaries) are well known.
From the time of naturalists to the present-day ecologists, there has been an enormous interest in understanding energy transfer in ecosystems, and research has increased notably since Lindeman (1942) coined the concept of trophic dynamics. This concept, along with the articles published by Odum, fostered a rapid progress of the use of the laws of thermodynamics in ecological studies.
Estuarine trophic dynamics
Trophic dynamics have been widely studied in estuaries, especially within the ichthyological community, but the role of other biological components in the flow of energy toward different trophic levels has only been suggested. Estuaries are habitats with an exceptional primary productivity that allows them to support a high abundance and biomass of fish; they are essential habitats in the life cycle of several teleosts, functioning as feeding, reproduction, and refuge areas. Fish communities in estuaries are very dynamic due to seasonal changes in abiotic factors: currents, water temperature, pH, freshwater inputs, evaporation, precipitation, and drastic changes in salinity. These changing and extreme conditions result in estuaries being characterized by few dominant fish species that are resident or typical of these areas. These species are classified as anadromous (fish that migrate from salt water to freshwater to reproduce) or catadromous (fish that migrate from freshwater to salt water to reproduce) (Kennish, 1990).
The trophic dynamics of estuaries are determined mainly by the trophic relationships of all biological components. The movement of nutrients between estuaries and marine communities occurs through the biota in a predictable sequence of trophic relations (nutrients). The trophic structure of communities is centered on the concept of the trophic web, which is the result of branched and/or linked trophic chains. It can be defined as the sum of all chains, with organisms grouped in categories or trophic levels, which consist of producers, consumers, and decomposers. These in turn are responsible for energy transfer. Therefore, at the base of the trophic chain are all the autotrophic organisms which fixate carbon through photosynthesis and provide energy to the primary consumers (heterotrophs) and so on through the upper trophic levels. Primary consumers serve as food for secondary consumers (herbivores), and these are a source of food for tertiary consumers (carnivores). Decomposers (saprophytic bacteria and fungi) assimilate dead plant and animal matter, transforming it in organic matter to get energy, while they release mineral nutrients that primary producers (autotrophs) use for growth. At each trophic level, approximately 80–90 % of potential energy is lost as heat, and this is a limiting factor that restricts trophic chain lengths to three to four trophic levels.
Estuarine trophic dynamic processes
The feeding habits of fish are important for several reasons. First, they reveal the trophic relations of the different species and indirectly bring to light aspects of the energy flow in estuarine communities. Second, they reveal the ecological relations between organisms, which can be used to better interpret the general dynamics of estuaries and to make recommendations for the appropriate administration of fisheries resources. The functioning of estuarine and coastal marine ecosystems depends mostly on using an external source of energy, solar radiation. A portion of this incident energy is transformed by photosynthetic processes (primary production) carried out by the phytoplankton (such as diatoms, dinoflagellates, coccolithophorids, cyanophytes, and silicoflagellates) and phytobenthos (red, green, and brown algae) (Lindeman, 1942). These components, along with detritus, constitute the main energy source for organisms in estuaries (Kennish, 1990). Zooplankton is comprised mainly of crustaceans such as copepods, as well as molluscs, coelenterates, and chaetognaths. The phytoplankton is consumed by zooplankton; about 50 % of phytoplankton is not consumed and goes into detritus. The zoobenthos consists of a wide range of organisms, mostly invertebrates such as sponges, crabs, echinoderms, polychaetes, snails, clams, and sea stars, which feed on the phytobenthos as well as on detritus. The zoobenthos is one of the main food sources for fish, which compose the most conspicuous and dominant community in estuaries. The abundance, biomass, and diversity of the ichthyofauna are regulated by the size of the estuary, the season (wet or dry), latitude (tropical and subtropical zones), and changes in abiotic factors (temperature, freshwater input, salinity changes, pH, currents within the estuary, and evaporation). These factors can lead to changes in the fish community structure through migration or immigration of less eurytopic species. However, most of the population is comprised by highly tolerant species that have a clear tendency toward an r strategy (Pianka, 1970).
In general, four fish feeding categories can be observed (Claro, 1994):
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Herbivores: This trophic category includes fish that feed exclusively on plants (algae). Fish from the Kyphosidae family are very well represented in estuaries.
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Detritivores: This group contains species that consume detritus, which consists of vegetal and animal material that has not been consumed and gets deposited on the benthos. The most typical fish family in estuaries is the Mugilidae family.
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Benthophagous: Fish that feed on small organisms that inhabit the substrate. Most fish species present in estuaries are found in this category (e.g., Mullidae and Chaetodontidae families).
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Carnivores: Consumers of animal prey, they feed mainly on fish and invertebrates. The most representative species belong to the families Lutjanidae, Serranidae, Haemulidae, Carangidae, and Balistidae.
A practical way to analyze the trophic dynamics of estuaries is by performing functional diversity studies that provide a rapid characterization of communities, reducing complex ecosystems into species groups (functional groups) with ecological equivalence, thereby facilitating comparative community studies (Root, 1967; Root, 2001).
Species that comprise a functional group are those that overlap in the highest number of variables in the multidimensional niche, being ecologically equivalent in function (Nagelkerken and Van der Velde, 2004). From the functional viewpoint, biological communities can be represented as a mosaic of functional groups, or as bricks with which communities are built (Krebs, 2003). The presence of multiple species (dominant and subordinates) within each functional group increases functional redundancy and contributes to the resilience of the estuary or ecosystem studied (Hooper et al., 2005).
With a higher number of functionally similar species, each with different responses to environmental factors, there is a higher probability of at least one species surviving possible perturbations (Hooper et al., 2005). If there is no functional redundancy, the loss of a single species could result in the complete loss of a functional group; therefore, at least one species per functional group is essential for ecosystem functioning (Díaz et al., 2005). Currently, due to urban and touristic development, anthropogenic impacts affect negatively coastal areas and especially estuaries, so that immediate and continued evaluations of these effects on the local fauna and flora are extremely important. The study of functional diversity is therefore an effective and precise tool to evaluate these effects.
The most important inputs for the development and evolution of trophodynamic perspectives over the last decades have occurred mainly in the field of aquatic ecology (Lindeman, 1942; Ulanowicz, 2004). The mass balance model ECOPATH with Ecosim has been the most extensively used tool for analyzing trophic webs (Polovina, 1984; Christensen and Pauly, 1992). This model describes quantitatively the energy flows (biomass) within food webs and facilitates an approximation to the structure and function of the ecosystem. Additionally, the model outputs allow the calculation of several ecological indicators of the state of the trophic web, such as ascendancy, surplus, and development potential (Ulanowicz, 1986; Libralato et al., 2006), that can be used to follow up an ecosystem’s development through time and to compare the ecological maturity of systems located in different parts of the world.
Summary
Estuarine trophic dynamics is one of the most complex processes of estuarine science. Its complexity lies in the high environmental heterogeneity and biodiversity of these coastal systems, which includes all trophic levels from the smallest autotrophs to the largest predators. The microbial decomposers must be considered as well. Among the most important components in the flow of energy in estuaries are fish communities due to the great diversity of finfish eating habits which has resulted in a large number of direct and indirect trophic interactions with other biotic components present within networks that structure and determine the energy dynamics of these valuable ecosystems.
Cross-references
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Moreno-Sánchez, X., Abitia-Cárdenas, A., Rodríguez-Baron, J.M., Uc, M.L., Riosmena-Rodríguez, R. (2016). Trophic Dynamics. In: Kennish, M.J. (eds) Encyclopedia of Estuaries. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-94-017-8801-4_267
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