The relative importance of mangroves and seagrass beds as feeding areas for residents and transient fishes among different habitats in Florida and Belize: Evidence from dietary and stable-isotope analyses

Study Number:

20

Author:

A. Vaslet, D.L. Phillips, C. France, I.C. Feller, C.C. Baldwin

Abstract:

In the western Atlantic region, the contribution of mangrove food sources to fish diets has been considered of more limited importance than previously expected due to the proximity of mangroves to adjacent potential food sources such as those in seagrass beds. To investigate the influence of different types of mangrove habitats on the relative contribution of mangrove and seagrass food sources infish diets, four mangrove habitats adjacent to seagrass beds were studied in Florida and Belize using gut-contents and stable-isotope analyses: mangrove fringe forests, basin mangrove, mangrove ponds and overwash mangrove islets. Carbon and nitrogen stable isotope compositions of 41 fish taxa and an array of potential primary (microphytobenthos, litter, seagrass leaves and their epiphytes, algae, plankton) and secondary (benthic invertebrates) prey were analyzed with SIAR mixing models to examine food source contributions in fish diets relative to habitat type. In all study sites, δ 13 C values of mangrove prey were significantly depleted relative to those from seagrass beds, allowing stable isotopes to provide reliable insights about origins of fish food. Seagrass prey located near basin mangroves in the Indian River Lagoon (IRL, Florida) had more negative δ13 C signatures than seagrass prey adjacent to fringing mangroves of the Florida Keys, suggesting that seagrass from the IRL incorporated dissolved inorganic carbon from mangroves. Contributions of mangrove and seagrass prey to fish diets were influenced by type of mangrove habitat and fish residency status. Resident species significantly relied on mangrove prey, whereas only four transients foraged in mangroves. Most transient fishes occurring in basin and fringing mangroves actively foraged in nearby seagrass beds, thus reinforcing the limited role of mangroves as fish foraging habitat for transient species. However, a shift in fish diet was observed for transient species from mangrove ponds, in which they relied on mangrove prey. In overwash mangroves (Belize), the enriched carbon signatures of fishes and the generally higher contributions of seagrass prey to fish diets suggest that fishes derived most of their food from seagrass beds. This trend was particularly highlighted for juvenile reef fishes that shelter in mangroves, but forage in nearby seagrass beds. These findings emphasize the importance of considering fish ecology (residency and life status) and type of mangrove habitat when assessing the contribution of mangrove prey to fish food webs in the western Atlantic region.

Main Results and Conclusions:

• Overview of the study: “The objective of this study was to assess the relative importance of mangrove and seagrass food sources in fish diets, taking into consideration several types of mangrove habitats in the western Atlantic region”(82).
• The results from this study show that seagrass beds play a larger role in sources of food for a majority of fish rather than mangrove habitat based on carbon content of primary producers and animal prey: “In all study sites, mangrove prey items were more 13C depleted than those from seagrass beds (t-tests, Pb0.001, Figs. 2a, 3a, 4a). In each study site, primary producers showed distinct carbon signatures, with lower values observed for mangrove litter (−28.9‰ to −27.1‰) and higher values for green seagrass leaves (−14.6‰ to −7.0‰) (t-tests, Pb0.001, Figs. 2a, 3a, 4a). In Belize, an increasing δ 13C gradient was observed between primary producers from fringing mangroves bordering the overwash islet (Be-MO: −28.9‰ to −16.5‰), seagrass beds adjacent to mangroves (Be-SG: −16.0‰ to −13.0‰, t‐tests, Pb0.001) and seagrass beds far from mangroves (Be-SGF: −8.9‰ to −7.0‰, t‐tests, Pb0.001) (Fig. 4a)…Even with some similar carbon signatures between mangrove and seagrass motile prey items (i.e., Majidae crabs from IRL-SG and Mysidacea from Be-MO), invertebrates from mangroves had in general more depleted δ 13C values compared to those from seagrass beds (t-tests, Pb0.001, Figs. 2a, 3a, 4a)”(86,87).
• This study emphasizes the importance of mangrove habitat for juvenile fish habitat rather than feeding grounds” “This study showed that most transient fishes (22 out of 31 species) from subtropical (IRL-MB, Keys-MF) and tropical mangroves (Be-MO) actively foraged in nearby seagrass beds, thus reinforcing the limited role of mangroves as fish foraging habitats for these species…It has been shown that structural complexity of mangrove prop roots attract juvenile reef fishes, which use this habitat more as shelters and nursery grounds than as foraging areas (Laegdsgaard and Johnson, 2001; Nagelkerken et al., 2010).”(91).
• In conclusion: “this study emphasizes the importance of considering different mangrove habitat types when investigating fish foraging grounds. SIA facilitated tracing of mangrove and seagrass food-source contributions in fish diets and revealed that 8 out of 10 resident and only 4 out of 31 transient species from basin, ponds, fringing and overwash mangrove sites derived a substantial proportion of their diets from mangroves. In Florida and Belize, these species were large roving fishes, zoobenthic or water column feeders that depend indirectly on organic matter from mangroves through the benthic–pelagic food web or secondary consumers. The remaining fish species, including juvenile reef fishes, occur in mangroves but appear to forage in adjacent seagrass habitats”(91).
• This article may be important to show the importance of both seagrass beds and mangrove habitat to fish species because of the connected role of the two ecosystems. If seagrass habitat is being destroyed, there may not be enough food in mangrove habitat to support a healthy ecosystem. Likewise, destruction of mangrove habitat may lead to the depletion of a species due to a lack of habitat for juveniles. 

Works Cited:

Laegdsgaard, P., Johnson, C., 2001. Why do juvenile fish utilise mangrove habitats? J. Exp. Mar. Biol. Ecol. 257, 229–253.

Nagelkerken, I., De Schryver, A.M., Verweij, M.C., Dahdouh-Guebas, F., van der Velde, G., Koedam, N., 2010. Differences in root architecture influence attraction of fishes to mangroves: a field experiment mimicking roots of different length, orientation, and complexity. J. Exp. Mar. Biol. Ecol. 396, 27–34.