Overall, our results suggest that organic enrichment had a negative effect on meiobenthic copepod diversity and taxa richness. We did not find a relation between the organic matter content and the coast position, but we found that the negative effect of nutrient enrichment on copepods is more pronounced in river mouths. Our results also suggest a synergic interaction between salinity and organic matter content; instead, temperature and pH seem not to be structuring the copepod community.
Regarding the negative effect of organic matter on copepod community our results differ to those found in previous research (e.g., Danovaro et al. 2000) since they found that the input of a river plume increased the organic matter concentration which led to an increase in copepod density. However, the study was carried out at the subtidal level where environmental conditions are more stable. In intertidal environments, large changes in temperature, salinity, and pH can affect the copepod community differently (Iwasaki 1999). Nevertheless, an increase on nutrients can cause hypoxia and De Troch et al. (2013) found that copepods were the most sensitive meiobenthic taxon. We did not assess whether the organic matter produced hypoxia but this scenario could explain the situation on the studied coasts. Other authors had reported similar results (Gee et al. 1985; Marcotte & Coull 1974; Sugumaran et al. 2009) in both field studies and mesocosms experiments, and they also associated that decrease in the sediment oxygen with the negative impact of organic enrichment.
On the other hand, the negative effect of organic matter on taxa richness is greater in sites near river mouths as can be seen in the slopes of Fig. 2B. These sites are exposed to a large change in environmental variables due to the influence of freshwater input which can strongly determine the occurrence and species composition (Giere 2009; Sandulli & Nicola 1991). Mu et al. (2002) found that river input influenced on sediment transport which caused variations in copepod community structure, with a less diverse community near the river mouth. It can be hypothesized that changes in environmental variables combined with organic pollution may cause a stressful environment producing the marked reduction of copepod taxa.
Our results also showed that the negative effect of organic matter on copepod richness was higher in sites with high salinities. Salinity strongly affects the community composition and diversity and generally meiobenthic species richness and abundance increase with increasing salinity (Coull 1988). Our results contrast with those found by other studies where a decrease in salinity is accompanied by a decrease in diversity (Broman et al. 2019; Lallias et al. 2014; Van Diggelen & Montagna 2016). One possible explanation could be that species that tolerate high salinities are more sensitive to organic enrichment. A detailed study of taxonomic composition and functional groups of species could shed light on these contradictory results. In addition, we did not find an interaction between temperature or pH and organic matter which agrees with other studies, where salinity is the most important environmental variable in the meiobenthic community structuration (Broman et al. 2019; Semprucci et al. 2019).
Contrary to our expectation, we did not find a relation between the organic matter content and the coast position. We did find different amounts of organic matter in different sites but we could not elucidate a pattern of distribution associated with the influence of the river. This could be explained by the low number of sampling coast positions since we took just three sampling points which could not be enough to evaluate the influence of the river. For example, Danovaro et al. (2000) and Marcotte & Coull (1974) found that the river caused the increase in organic matter, but considering a larger number of sampling points.
Meiobenthic copepods constitute a neglected taxonomic group in Argentina and particularly, for Buenos Aires coasts the knowledge on this group has remained limited to a handful of papers (Ramírez 1971; Sciberras et al. 2014; 2018; 2021). Even less is known about the state of conservation of the Argentinean coasts and how anthropogenic activities impact meiobenthos. Although ecological interpretations are difficult to make without an extensive background environmental data set and a detailed study of taxonomic composition and functional groups, we believe this work provides insights on the structure of the meiobenthic copepod community and the possible impacts of organic enrichment on coasts. The exact nature and potential interaction of the measured variables with the diversity of copepods cannot be univocally elucidated by the descriptive approach and limited temporal scales of the present study, but the findings provide relevant baseline information for future works.