Assessing benthic ecological status under impoverished faunal situations: A case study from the southern Gulf of Mexico

doi:10.1016/j.ecolind.2018.04.048

Ecological Indicators

Volume 91, August 2018, Pages 679-688

https://doi.org/10.1016/j.ecolind.2018.04.048 Get rights and content

Abstract

Biological indices such as AZTI’s Marine Biotic Index (AMBI) and multivariate AMBI (M-AMBI) have been used in monitoring programs worldwide to assess the benthic ecological status (ES) of transitional and coastal waters. However, their reliability is reduced under faunal impoverishment and defaunation, low abundance/number of taxa, and/or high percentage of taxa not assigned or mis-assigned to ecological group, which imply removing these data from the analysis. In order to avoid loss of robustness when these situations are met, here we propose an approach based on decision criteria that utilize these indices together with environmental and contaminants data. Our area of study is the southern Gulf of Mexico seafloor, where a survey was carried out during the rainy season across a sampling grid of 75 sites. To achieve this, we first distinguished homogeneous groups of sites and then defined states (‘good’, ‘fair’, and ‘bad’) from three sediment quality elements: benthic indices, environmental data by quartile values, and contaminant concentrations in the context of sediment quality guidelines. Overall, 69% of sampling sites showed low abundance and defaunation, mainly at sites located on the continental shelf, where most of them ranged from ‘moderate’ to ‘poor’ ES, principally by ‘fair’ environmental data and secondly, oil-related disturbance. Conversely, sites located near the mouths of rivers and coastal lagoons recorded the highest abundance and showed diverse sensitivity levels, ranging from ‘high’ to ‘bad’ ES. In conclusion, the use of this states-based approach allowed us to support and interpret the results of AMBI and M-AMBI, since their values were related to environmental and contaminants data. This approach may be useful in many contexts to avoid the loss of data when assessing the ES of the seafloor under defaunation or low abundance conditions.

Introduction

Macroinfauna species are often used as bioindicators of natural disturbance and anthropogenic pollution of aquatic systems (Borja et al., 2000). These organisms are used in monitoring programs because they are relatively sedentary, they have relatively long life-spans and they participate in cycling nutrients and transference of energy to top trophic levels (Pearson and Rosenberg, 1978, Rosenberg, 2001). Their ecological responses allow them to be classified in a range from sensitive to opportunistic species, and their abundances have been used in marine biotic indices aimed at assessing benthic community health. For example, one application is the Marine Biotic Index (AMBI) developed by AZTI (Borja et al., 2000) along with the multivariate AMBI application (M-AMBI) (Borja et al., 2004b, Muxika et al., 2007).

The European Water Framework Directive (WFD) includes AMBI and M-AMBI methodologies to assess the ecological status of macroinvertebrates in transitional and coastal waters, as well as other biological (e.g. phytoplankton, macroalgae, fishes), physicochemical (e.g. salinity, temperature, nutrients, dissolved oxygen, contaminants) and hydrodynamic (e.g. waves, tides) elements (Borja et al., 2009). Among the biological elements, macroinfauna is of major importance. If for example, macroinfauna has a ‘moderate’ status and the remainder of the elements have a ‘high’ status, the ecological status should be ‘moderate’, after the ‘one out, all out’ principle (Borja et al., 2003, Borja et al., 2004b). Part of this approach is used by Borja et al. (2009) as a decision-tree to integrate different elements into a unified ecological status assessment.

AMBI and M-AMBI are sensitive and representative indices (Borja et al., 2015), but their reliability is reduced under faunal impoverishment and defaunation situations, i.e. sediment samples with low abundance/number of taxa (1–3) and/or a high percentage of taxa not assigned (>20%) or mis-assigned to an ecological group, which imply that sites with these characteristics must be removed from the analyses (Borja et al., 2004a, Borja and Muxika, 2005, Checon et al., 2018).

In some benthic areas that are naturally impoverished (i.e. coastal lagoons, dynamic beaches, oligohaline stretches), some authors (e.g. Dauvin, 2007, Munari and Mistri, 2008) have cautioned against the use of benthic indices, and alternate indices based on higher taxa have been proposed for use in such environments (Munari et al., 2009).

AMBI and M-AMBI have been considered as the most suitable biotic indices to assess ecological status, both in the southern (Granados-Barba et al., 2009, Domínguez-Castanedo, 2012) as in the northern Gulf of Mexico (Gillett et al., 2015, Pelletier et al., 2018). However, some areas have faunal impoverishment and even defaunation, which reduce the reliability of these indices. In order to avoid loss of data when calculating ecological status under these circumstances, the objective of this study was to develop an approach based on decision criteria considering benthic indices, environmental and contaminant data in a case study from the southern Gulf of Mexico (sGM).

Section snippets

Study area and sampling design

The study was carried out in the sGM between 18°9′29.99″–22°45′ N and 96°00′–88°00′ W (Fig. 1). The oceanographic features of this region are regulated by: (i) surface runoff into the southwestern Gulf of Mexico and ground water discharges in the Yucatan Peninsula area influence; (ii) the circulation pattern, which is dominated by wind conditions—during spring and summer the Lazo Current reaches maximum speed in a south to south-west direction, and during autumn and winter the flow reverts to a

Sedimentary environments

Based on sediment texture, three sedimentary environments were distinguished: terrigenous, transition and carbonated, principally identified by the differences in carbonate content (χ² = 62.1; d.f. = 2; P < 0.001), and secondly, by clay (χ² = 19.1; d.f. = 2; P < 0.001) and silt content (χ² = 16.1; d.f. = 2; P < 0.001) (Fig. 3). Thus, we used a range of carbonate content (20.9–76.9%) to show the sediment texture change along the transition zone from calcareous to terrigenous (Table 2). In

Discussion

We propose decision criteria that consider the benthic indices jointly with environmental and contaminant data when faunal impoverishment and defaunation situations occur at sites without high disturbance conditions. We defined states through methodologies tested in tropical areas exposed to several anthropogenic pressures (Venturini and Tommasi, 2004, Herrera-Silveira and Morales-Ojeda, 2009, Borja et al., 2015). The relative weight of these three sources of data has to be equal because

Conclusions

This is the first attempt to analyze benthic indices under faunal impoverishment and defaunation situations, using data recorded in the sGM. To accomplish the macroinfauna ecological status assessment, we classified the study area based on sedimentary environments determined in accordance with sediment texture, depth of the water column and benthic composition. Then we assigned states to each environment, which are not value judgments on ecological consequence, but represent instead

Acknowledgments

The data used in this study resulted from the ‘Programa de Monitoreo Ambiental del sur del Golfo de México,’ carried out jointly by PEMEX Exploración y Producción, Regiones Marinas and Cinvestav Unidad Mérida; in particular, we are grateful to the laboratories of ‘Bentos’, ‘Química Marina’, and ‘Geoquímica’ which generated these data. Thanks are also due to Maria Teresa Herrera-Dorantes (Laboratory of Benthos, Cinvestav) for her support in sampling in the field, laboratory procedures and

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Original ArticlesAssessing benthic ecological status under impoverished faunal situations: A case study from the southern Gulf of Mexico

Abstract

Introduction

Section snippets

Study area and sampling design

Sedimentary environments

Discussion

Conclusions

Acknowledgments

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Original Articles
Assessing benthic ecological status under impoverished faunal situations: A case study from the southern Gulf of Mexico