Shelf and deep-water benthic macrofauna assemblages from the western Gulf of Mexico: Temporal dynamics and environmental drivers

Highlights

• Shelf and deep-water macrofauna show seasonal and interannual variation in both species and functional trait assemblages.

• Changes in assemblage composition were correlated with environmental conditions, according to depth range.

• Variation on the shelf benthic macrofauna was associated with bottom oxygen, POC flux, PAHs and lead concentration.

• Changes on deep-water assemblage composition were highly correlated with POC flux.

Abstract

Shelf and deep-water soft-bottom macrofauna were explored in the western Gulf of Mexico in terms of species and functional trait assemblages. Their variation was analysed as functions of depth and time, and the relationship with sea-bottom environmental conditions was examined to disentangle their association with potential environmental drivers. Four consecutive cruises (two per year, at the end of the dry and rainy seasons) were performed during 2016–2017 at 27 fixed stations distributed from 42 to 3565 m depth. Changes in macrofauna composition were tested considering species and functional trait assemblages. Environmental variables associated with sediment features (i.e., grain structure, organic matter, pH, redox), oceanographic conditions (i.e., temperature, dissolved oxygen, particulate organic carbon flux) and potential contaminants (i.e., hydrocarbons and metals) were analysed to identify potential drivers that would shape the structure of macrofauna assemblages. The results suggest that the structures of both species and functional trait assemblages change according to depth and show temporal variation in composition at seasonal and interannual scales. The effect of temporal variation represented about 12% of total variation in the assemblages (11.4 for species and 12.5% for functional-traits). Different patterns of spatial and temporal variation between shelf and deep benthic communities were observed. Variation in species assemblages on the shelf were related to the variation in lead, polycyclic aromatic hydrocarbons and the fine sand ratio. In the deep benthos, particulate carbon flux showed high correlation with the spatial and temporal variation in species assemblage. In the deep benthos the changes in the species assemblage between the dry and the rainy seasons and the interannual variation were highly correlated with particulate organic carbon input in the area.

Introduction

The shelf and deep-water benthic habitats of the Gulf of Mexico (GoM) are currently affected by a number of environmental conditions that drive temporal and seasonal variation of their assemblages (Rowe, 2017). At the north of the GoM the variation of shelf benthic abundance, biomass and diversity had been associated with the seasonal variation of summer and winter climatic seasons (Montagna and Harper, 1996; Brooks et al., 2006). At the western side of the GoM, the variation in shelf benthic biomass had been associated with seasonal variation of riverine runoffs and cold fronts due to their effects on phytoplankton enrichment and the increase in mixed layers (Escobar-Briones and Soto, 1997). At the southern end of the GoM the variation in shelf benthic assemblages had been associated with seasonal variation of dominant climatic conditions (Hernández-Arana et al., 2003), producing marked temporal dynamics (Hernandez-Avila et al., 2020).

Concerning the deep-water habitats, there is a consensus suggesting that the energy, in terms of organic matter input from the surface, is a major driver of variation in benthic assemblages at different spatial and temporal scales (Baguley et al., 2006; McClain et al., 2012; Watling et al., 2013; McClain and Rex, 2015; Woolley et al., 2016). There is also evidence supporting the hypothesis that the assemblage structure of the northern GoM deep-water benthos is associated with the input of organic matter from shallow waters, affected by the influence of the Mississippi river (Wang, 2004; Baguley et al., 2006; Haedrich et al., 2008; Rowe et al., 2008; Wei et al., 2010; Rowe, 2017). In addition, the large number of chemosynthetic habitats (such as cold seeps and brine pools) along the GoM play a role in local chemosynthetic production of the benthic systems (Pequegnat et al., 1990; Cordes et al., 2009; Demopoulos et al., 2018).

Additionally, there is an increasing amount of evidence supporting the occurrence of a seasonal variation of deep-water benthos at both population and community level (Pequegnat et al., 1990; Smith et al., 2006; Mincks and Smith 2007; Glover et al., 2008). These patterns had been associated with the seasonal variation of primary production and carbon input (Tyler, 1988; Gooday and Turley, 1990; Glover et al., 2010). Recent evidence from the northern GoM shows a seasonal and interannual variation of deep-water benthic communities associated with variation of organic matter imported from the surface (Wang, 2004; Reuscher and Shirley, 2017; Montagna et al., 2020). However, a general response could not be assumed for the entire GoM since the processes associated with the carbon pump in the deep sea, and their temporal variation, change at the regional scale (Glover et al., 2010).

Identifying the temporal scales of variation and disentangling the natural variation from the response to anthropogenic stress is a major challenge in a threatened ecosystem such as the GoM. Here we explore the patterns of species and functional trait diversity of benthic macrofauna on the continental shelf and deep benthos from the western GoM, their variation as functions of the depth and time and their relationship with sea-bottom environmental conditions including sediment metals and hydrocarbons and particulate organic flux. The aim of this study was to estimate the variation in species diversity and assemblage composition, based on both species and functional traits, along depth in the western GoM (the Perdido Fold Belt and adjacent areas) and test the hypothesis of temporal (seasonal and interannual) variation in shelf and deep-water macrofauna associated with environmental conditions.