Stable Isotopes Clearly Track Mangrove Inputs and Food Web Changes Along a Reforestation Gradient

Assessment of aquatic food web and trophic niche as a measurement of recovery function in restored mangroves in the Southern Gulf of Mexico

Mangroves are coastal wetlands with high biodiversity and productivity, with great interaction with coastal environments. In the face of worldwide mangrove loss, restoration projects attempt to recover ecosystem composition and functioning over time. Our objective was to examine and compare the food webs in mangrove areas with different restoration times and in a reference mangrove in Términos Lagoon, Mexico. We estimated the trophic structure, identified the carbon resources that maintain aquatic consumers through the analysis of stable isotopes, and compared the trophic niche of the restored mangroves with the reference mangrove. We analyzed environmental variables, trophic structure, and contributions of resources during three seasons: rainy, dry, and "nortes". Environmental changes and food structure changed in response to regional seasons. Bayesian mixing models indicated that food webs varied seasonally as a response to the primary productivity developed at Términos Lagoon. As expected, the assimilation of C₃ plants in the reference mangrove was highest, as a primary ("nortes" season) and secondary resource (dry and rainy seasons). The restored mangroves depended mainly on allochthonous resources (seagrass, epiphytes, and phytoplankton). The assimilation of these resources highlighted the importance of connectivity and the input of sources of carbon from nearby coastal environments. Trophic niche analysis showed that the area with longer restoration time was more similar to the reference mangrove, which is evidence of the importance and efficacy of the restoration process, as well as the restoration of the ecosystem function over time.

Food web dynamics in the mangrove ecosystem of the Pearl River Estuary surrounded by megacities

Global recognition that mangroves support coastal ecosystem services has increased; however, studies on trophic dynamics in mangrove ecosystems remain limited. We seasonally analysed the δ¹³C and δ¹⁵N of 34 consumers and 5 diets to elucidate the food web dynamics in the Pearl River Estuary (PRE). Fish had a large niche space during the monsoon summer, reflecting increased trophic roles. In contrast, the small niche space of benthos over seasons reflected consistent trophic positions. Consumers mainly utilized plant-derived organic matters in the dry season and particulate organic matters in the wet season. The present study with literature reviews revealed characteristics of the PRE food web with the depleted δ¹³C and enriched δ¹⁵N, indicating a high contribution of mangrove-derived organic carbon and sewage input, particularly in the wet season. Overall, this study confirmed the seasonal and spatial trophic dynamics in mangrove forests surrounding megacities for future sustainable mangrove ecosystem management.

Are stable isotopes an efficient tool for tracking the effect of anthropogenic activities on mangrove food web structure and function?

Understanding and connecting the impact of anthropogenic activities on mangrove food webs is a research challenge. Has research on the subject been able to find answers using stable isotopes? The present opinion paper analyzed the utility of stable isotopes in tracing the impact of anthropogenic activities on mangrove food webs and if the research questions raised could be answered using these chemical markers. Representative research papers (16) focused on the use of stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S, δ²H, δD,206Pb/207Pb, and 208Pb/207Pb) to evaluate the effect of anthropogenic activities (Sewage discharge, timber harvesting-deforestation, metallurgical activities, hydrological disruption, aquaculture ponds, and urban development) on mangrove food webs were selected. Each article included at least one group of consumers (invertebrate or fish). Publications only focused on water quality or primary producers were not included. Most studies managed to determine the effect of the anthropogenic activities on the food web's stable isotope values. Based on the above, we concluded that these markers are an effective tool to determine affectation patterns on the structure and function of mangrove food webs. The results obtained herein facilitate the correct management of mangroves and their derived resources.

Compound-specific stable isotope analysis of amino acid nitrogen reveals detrital support of microphytobenthos in the Dutch Wadden Sea benthic food web

The Wadden Sea is the world’s largest intertidal ecosystem and provides vital food resources for a large number of migratory bird and fish species during seasonal stopovers. Previous work using bulk stable isotope analysis of carbon found that microphytobenthos (MPB) was the dominant resource fueling the food web with particulate organic matter making up the remainder. However, this work was unable to account for the trophic structure of the food web or the considerable increase in δ15N values of bulk tissue throughout the benthic food web occurring in the Eastern regions of the Dutch Wadden Sea. Here, we combine compound-specific and bulk analytical stable isotope techniques to further resolve the trophic structure and resource use throughout the benthic food web in the Wadden Sea. Analysis of δ15N for trophic and source amino acids allowed for better identification of trophic relationships due to the integration of underlying variation in the nitrogen resources supporting the food web. Baseline-integrated trophic position estimates using glutamic acid (Glu) and phenylalanine (Phe) allow for disentanglement of baseline variations in underlying δ15N sources supporting the ecosystem and trophic shifts resulting from changes in ecological relationships. Through this application, we further confirmed the dominant ecosystem support by MPB-derived resources, although to a lesser extent than previously estimated. In addition to phytoplankton-derived particulate, organic matter and MPB supported from nutrients from the overlying water column there appears to be an additional resource supporting the benthic community. From the stable isotope mixing models, a subset of species appears to focus on MPB supported off recycled (porewater) N and/or detrital organic matter mainly driven by increased phenylalanine δ15N values. This additional resource within MPB may play a role in subsidizing the exceptional benthic productivity observed within the Wadden Sea ecosystem and reflect division in MPB support along green (herbivory) and brown (recycled/detrital) food web pathways.

Trophic structure of fish communities in mangrove systems subject to different levels of anthropogenic intervention, Tropical Eastern Pacific, Colombia

Mangroves are important ecosystems of tropical and subtropical shorelines. Anthropogenic activity decreases their habitat quality, affecting structural and functional trophic features. We hypothesized that higher levels of anthropogenic intervention generate diversity loss and modify the trophic structure of tropical mangrove fish communities. We compared the taxonomic and isotopic (δ¹³C/δ¹⁵N) composition, abundance, trophic position, and isotopic niche of fish communities from three mangrove systems with different anthropogenic intervention levels in the Colombian Pacific. Non-parametric statistical tests and a Bayesian approach were used to analyze data. A total of 1254 specimens belonging to 23 families, 25 genera, and 30 species were identified, presenting higher abundance (821) in moderate anthropogenic intervention level mangrove (Moderate-AIL), with high dominance of one species (Lile stolifera). The low anthropogenic intervention level mangrove (Low-AIL) was the second in abundance (291) but exhibited a greater number of species (23), while the high anthropogenic intervention level mangrove (High-AIL) presented the least abundance (142) and species number (17). The isotopic composition ​​reveals that Moderate and High-AIL mangroves presented enriched ¹³C and ¹⁵ N compared to Low-AIL (~ 2 to 4 ‰). Mean trophic position (TP) of communities was slightly higher in the more intervened systems (~ 1 to 2 orders of magnitude), as well as in specific species (Centropomus spp.). Isotopic niche width (TA and SEAc) was greater in High-AIL (41.1 and 9.2), more than doubled compared to Moderate-AIL (33.0 and 4.1). In High-AIL isotopic niche width increased, indicating lesser availability of prey and basal resources. The results obtained in this study support the proposed hypothesis and, suggest that anthropogenic intervention modifies diversity and food webs dynamics, affecting the transfer of matter and energy from macrotidal tropical mangroves to coastal ecosystems. However, it is recommended to be careful concluding differences based exclusively on the anthropogenic intervention level, since it is widely documented that mangrove settings also influence the analyzed trophic parameters.