The study was performed in the southeastern coast of the Urabá Gulf (Fig. 1). Located near the Colombia-Panama border, the gulf is a north-facing embayment that represents the southernmost region of the Caribbean Sea. The gulf is home to the most developed mangrove forests in the Colombian Caribbean, which are probably the most productive in the Americas (Riascos and Blanco-Libreros 2019). Fringe forest is the dominant physiographic type of mangroves in the region, which mostly comprise monospecific stands of Rhizophora mangle while R. mangle, Laguncularia racemosa and Avicennia germinans occur in basing mangroves (Urrego et al. 2014). The gulf is part of the Chocó-Darien Global Ecoregion, a globally recognized biodiversity hotspots prioritized for conservation due to the high levels of biodiversity and endemism (Fagua and Ramsey 2019). Despite this, the coalescence of outrages and conflicts that characterized the aftermath of European invasion in Latin America is epitomized in this region. The region witnessed the rose and dead of the oldest funded Spanish city of the Americas in terra firma (Sarcina, 2017), the spread of African-descendant peoples that displaced indigenous groups after slavery abolition and the growth of coca cultivation and the linked problem of illegal armed groups that turned the region into a major trafficking illegal immigration corridor. These historic processes have resulted in a region displaying a complex mosaic of land covers, ethnic groups and legal and illegal economic activities.
Mangroves in Turbo Bay have been characterized as “peri-urban” because they are structurally and functionally affected by its proximity to the Turbo port city (Blanco-Libreros and Estrada-Urrea 2015). During the last 15 years, the number of homes in the Turbo District increased by 18.12%, most of them concentrated in Turbo city that currently is home to 48,787 people (DANE, 2018). Moreover, the ongoing development of major port facilities will further boost urban expansion and the associated pressures on mangrove forests in coming years. In turn, mangroves at El Uno Bay have been cleared for expanding lands for agriculture (mainly comprising plantain crops) and cattle ranching, a typical example of a rural-agricultural transition (Blanco-Libreros and Estrada-Urrea 2015). The bay is a coastal lagoon whose formation is linked to the evolution of the Turbo River delta since the transfer of its mouth to this region in the mid-20th century (Blanco-Libreros et al. 2013, Alcántara-Carrió et al. 2019).
This work builds on a previous work on the structure of macrobenthic communities associated to prop roots of R. mangle performed by García and Correa (2006), which was latter published by García and Palacio (2008). They sampled six prop roots in the eastern, western and northern zones of each bay between September and December 2005. They found that diversity of macrobenthic communities did not significantly change trough time or zones. Moreover 12 species comprised 90% of the total abundance and these species were found in all sampling points through the study period. Hence, we performed a single sampling in June 2021, taking ten R. mangle prop roots in the same zones (east, north, west) in each bay (Fig. 1). Following the criteria established by García and Palacio (2008), roots were selected by i) belonging to mature trees (≥ 10cm in diameter at breast height), ii) having a significant portion submerged into the water and iii) harboring easily seen sessile organisms. The roots were cut at the high-tide mark and immediately stored in labeled plastic bags. Additionally, the following factors related to anthropogenic disturbances were registered: trampling (the number of human footprints), logging (number of trees cut), litter (number of litter items) and urban structures (number of urban structures, i.e. houses, roads, peers, etc.). All these counts were performed by a single dedicated observer in the area surrounding each sampling point.
Samples were immediately taken to the Marine Ecology laboratory (Universidad de Antioquia, Marine Science Campus in Turbo), refrigerated at 3–5°C and processed within the next 12 hours. Roots were weighted and placed on plastic trays, cut into small parts and dissected. Observed macroinvertebrates were removed and stored in alcohol. Oysters in particular were carefully reviewed under a stereoscope to record attached organisms. Plastic bags and root pieces were washed and sieved through a 250-micron mesh sieve. The retained material was stored in labeled plastic jars with 95% ethanol for further analysis. The samples were sorted under a stereomicroscope and the resulting organisms identified to the minimum possible taxonomic level. Following the criteria and definitions used by the Convention on Biological Diversity on invasive alien species (https://www.cbd.int/invasive/terms.shtml), all taxa identified to the species level were categorized as:
Indigenous species: a species living within its natural range (past or present) including the area which it can reach and occupy using its natural dispersal systems.
Alien species: a species introduced outside its natural past or present distribution.
Invasive-alien species: an alien species whose introduction and/or spread threaten biological diversity
Finally, a species that was not demonstrably native or introduced based on current knowledge was classified as a cryptic species.
As sampled prop roots had distinct weight, the abundance of species was calculated as the number of individuals per gram of root. These data were organized in biological (species abundances per root) or environmental (anthropogenic disturbances in each sampling point) matrices. Abundance data were square-root transformed to balance the contribution of abundant and rare species in further analysis, thus accounting for the fact that some fast-moving animals had a chance to escape during samplings, as opposite of sessile animals. Data on environmental factors were first normalized (subtracting the mean and dividing by the standard deviation for each variable) to account for the different scales among variables. The Bray-Curtis dissimilarity index was later estimated from abundance data for each pair of samples in the matrix and Non-metric multidimensional scaling (nMDS; Clarke and Gorley 2006) was used to build ordination plots of the structure of macrobenthic communities in mangrove roots for each zone and bay. In turn, Euclidean distances were calculated between sampling points to describe abiotic differences among zones in each bay, using bi-dimensional plots of Principal Component Analyses.
To test for changes in the structure of epibenthic macrofauna associated to roots between zone (east, north, west) and bays (El Uno, Turbo) we used a two-way ANOSIM test. This approach performs a permutation test of the null hypothesis of no differences among a priori defined groups of samples, based on the ranks of the sample dissimilarity matrix (Somerfield et al. 2021). This preliminary analysis confirmed that there were no significant differences between zones.
To assess our hypothesis on changes in the structure of epibenthic macrofauna associated to roots, samples from each zone were pooled and treated as replicates. A crossed two-way ANOSIM test was used to test for differences between periods (2005–2021) and bays (El Uno – Turbo). For samples found to be significantly different, the Similarity Percentage Analysis (SIMPER) implemented in PRIMER software was used to evaluate which species contributed most to the differences between periods and bays. This biota was further characterized according to size and origin (native/non indigenous) to discuss our findings. A significance level of α = 0.05 was chosen for all the tests performed. All multivariate analyses were performed
using PRIMER v.6 software (Clarke and Gorley 2006).