Nutrients transported by herons and spoonbills subsidize the environment and the entire trophic web where they breed. This conclusion was based on analyses in several dimensions - spatial, temporal and along the trophic chain - and in different compartments of the swamp forest ecosystems of the colony compared to a control, noncolony, area. This study demonstrates that the colony environment is enriched in 15N and 13C compared to the control environment, although this occurs to a lesser degree for 13C (spatial dissipation). Resources remain in the environment even after the waterbird-breeding season, demonstrating that the nutrient load deposited during spring-summer is a key resource for the trophic web of the swamp forest long afterwards (temporal dissipation). In addition, vertical analyses along the trophic chain showed that estuarine subsidies reach higher trophic levels. However, there are other important factors, such as the influence of trophic guild and habitat feeding. Omnivorous-detritivorous invertebrates, for example, tended to be more enriched than other invertebrate groups. These results may indicate that because they are detritivorous and consume available soil matter, they directly access the matter transported to the colony by waterbirds, assimilating more estuarine matter compared to other invertebrate guilds. Furthermore, migratory birds with broad habitats (MBH) showed lower isotopic values and lower assimilation of estuarine matter than other bird groups. This indicates that because they are migratory and not limited to the colony forest, they use less enriched food resources, and therefore their tissues were less isotopically enriched compared to other bird groups in the colony forest.
The δ15N and δ13C values in the organisms and soil of the colony environment were higher than those of the control environment. Thus, the results indicate that the presence of waterbirds isotopically enriches the colony forest. Furthermore, the use of carbon isotopes as a marker of matter flux in estuarine environments is rarely used and often not recommended (e.g., Spano et al. 2014). This is because estuarine environments constantly receive terrestrial matter that flows into the water, causing values in the environment to approach terrestrial values (Spano et al. 2014). However, in the Lagoa dos Patos estuary, the presence of C4 plants (phanerogams, tidal flats) contributes to the enrichment of 13C, resulting in significantly higher mean values in the estuary (Garcia et al. 2007; Pereyra et al. 2016). Therefore, in the present study, significant differences in δ13C were found between colony soil values compared to the control, thus demonstrating its importance as a natural marker complementary to the nitrogen isotope. The results therefore demonstrate that it is possible to detect 13C enrichment generated by the transport of estuarine matter into terrestrial environments, even when the receiving environment is highly productive, as in swamp forests. Furthermore, regular heron breeding at the site, year after year, can lead to saturation of the environment, making it isotopically well marked.
Similarly, δ15N values in soil, vegetation, invertebrates and birds in the colony environment showed higher values than those in the control area. Other studies conducted in oligotrophic receptor environments show that bird colonies, through the deposition of guano and matter, tend to enrich the receptor environment (Polis et al. 1997; Kolb et al. 2010; Caut et al. 2012; Savage 2019). In the present study, it was shown that enrichment also occurs in a productive receptor environment, such as swamp forests. This is likely due to the intensity of loading, with recurrent breeding at the same site, mediated by large and numerous waterbirds. These results indicate enrichment by guano, tissues of birds and their prey, from estuarine areas, or nutrients with high δ15N values in the swamp forest, from soil to land birds (Fig. 2). The isotopic values of the colony organisms were positioned in the upper region of the graph compared to the control environment (Fig. 4a, c), reinforcing the enrichment by 15N in this environment.
Because they occupy high trophic levels, predatory waterbirds have excreta enriched in 15N (Caut et al. 2012). Thus, guano- and deposited nitrogen-enriched matter are accessed by organisms at low trophic levels in the terrestrial environment, generating an upwards enrichment cascade across all higher trophic levels. Despite this, this study indicates that this deposition is widely distributed throughout the colony region, as everything from soil to vertebrates appears enriched. This spatial distribution in the colony environment occurs through biotic and abiotic vectors (Green and Elmberg 2014; Schindler and Smits 2016; Griffiths et al. 2018; Gaiotto et al. 2020), with water-saturated soil likely playing a key role, similar to water runoff on oligotrophic tropical islands (e.g., Sánchez-Piñero and Polis 2000; Gaiotto 2018).
Soil, invertebrates, plants, and terrestrial birds maintained or increased values even during the winter without the presence of the colonial waterbirds. This indicates that the pulse is so intense that the enrichment of the environment persists in the soil and vegetation. Similar studies on marine islands also indicate that once the vegetation is isotopically enriched, the marine signature persists in terrestrial consumers, even during periods without the presence of the colonial seabirds (Stapp and Polis 2003; Caut et al. 2012; McLoughlin et al. 2016).
For δ13C values, only land birds had a negative effect during the winter, although the effect shown was smaller than that shown for δ15N. It is expected that with the pause in the matter input pulse, the enrichment loses strength and does not reach the higher trophic levels, as is the case for birds. However, for both isotopes, there is an indication that outside of reproductive periods, the soil and vegetation allow organisms in the environment to continue utilizing matter of allochthonous origin. Other studies have indicated that guano carried by birds has the ability to enrich producers in the environment (Shatova et al. 2017; Savage 2019). However, plants obtain their carbon from atmospheric air, and therefore 13C enrichment does not affect producers but rather organisms that feed on the carcasses of herons that die in the colony and the food scraps transported from the estuary. The δ13C values of these organisms are derived from their feeding and reflect the values at the base of the trophic chain (Craig 1953; DeNiro and Epstein 1978). Therefore, since vegetation carbon values are independent of allochthonous input, carbon enrichment ultimately has a limited influence on the trophic web. However, even during winter, the colony shows higher carbon values compared to winter in the control zone, demonstrating the persistence of temporal enrichment by waterbirds.
Furthermore, the results demonstrated the depletion of 13C in invertebrates in 2018 compared to 2019. The study results may indicate that the dissipation of matter during winter 2019 may have been influenced by El Niño, when longer periods of precipitation occur (Odebrecht et al. 2017). The region is composed of palustrine forest, which has inundation characteristics at certain times of the year (Waechter 1990). Other studies corroborated that part of the nutrient distribution in the environment is driven during flood and rainfall periods (Schindler and Smits 2016; Li et al. 2017). This may indicate that in addition to transport by biotic vectors, abiotic factors such as rainfall and inundation periods also contribute to the temporal dissipation of transported nutrients, similar to oligotrophic environments (Gaiotto 2018).
Vertical dissipation and trophic guilds
Estuarine-derived nutrients were identified in the soil and tissues of all groups of organisms analysed in the colony, and 15N enrichment was higher in the colony than in the control area. This indicates a link between the bird colony and the local trophic web, with the latter being dependent on allochthonous nutrient inputs mediated by waterbirds. Both herbaceous plants and trees were enriched by the input of estuarine matter, with no significant differences between them. Only the fern group showed no significant enrichment in the colony environment compared to the control. However, this group of plants was collected at the edges of the colony region, which may indicate that as the distance from the core region where a higher concentration of nests occurs, the enrichment loses intensity. Similarly, other studies have also shown that distance from the source of matter, whether rivers, ocean, or colonies, is important in regard to the dissipation of the isotopic signature (Caut et al. 2012; Korobushkin 2014; Gaiotto 2018).
In primary consumers, the average enrichment was maintained compared to soil and vegetation. However, when separated into trophic guilds, we observed differences in the assimilation of estuarine matter. The omnivorous invertebrates (ants) of the genus Camponotus, which feed predominantly on plant matter, invertebrates, and fruits (Ronque 2013), showed lower levels of estuarine matter assimilation during winter (Fig. 5a). A similar pattern occurred with carnivorous invertebrates (Fig. 5c), which do not feed directly on producers but have isotopic signatures of their prey (DeNiro and Epstein 1978). In the guild of omnivore-detritivores (garden armadillos), the pattern also emerged in the colony but not in the control environment, where there was an increased probability of assimilation during winter. Another difference found between invertebrate groups and the omnivores-detritivores was the higher probability of assimilation of estuarine matter in both seasons in the colony in comparison to the control (Fig. 5b). These results suggest that omnivorous-detritivorous armadillos feed on sources other than matter from local producers and organisms, such as dead nestlings and food scraps carried by waterbirds (Markwell and Daugherty 2002).
On the other hand, at higher trophic levels, land birds in the colony environment had 15N enrichment in their blood. Except for omnivorous birds during winter, the assimilation in insectivorous and omnivorous bird guilds had a higher probability and assimilation of estuarine matter in the control environment compared to the colony (Fig. 5d, e). These results can potentially be explained by the fact that the forest boundaries of the control environment are closer to the estuary than the colony environment. Previous studies have pointed out that emerging invertebrates in aquatic environments are important sources for terrestrial consumers (Nakano and Murakami 2001; Schindler and Smits 2016; Recalde et al. 2021). Therefore, we suggest that although the control area does not receive any heron-mediated estuarine matter input, resident animal groups utilize external food resources associated with the estuarine environment found at the forest margins.
Finally, migratory birds, with a broad foraging habitat (MBH) and large movements, i.e., they do not feed exclusively within the colony forest, tended to be less enriched than photophobic, forest-restricted bird groups. This group had lower 13C and 15N enrichment, suggesting that the length of time organisms had been present in the environment also influences the level of enrichment in tissues due to the shorter exposure time to enriched foods from the colony area (DeNiro and Epstein 1978; Zupcic-Moore et al. 2017).