Low-pH Freshwater Discharges Drive Spatial and Temporal Variations in Life History Traits of Neritic Copepod Acartia tonsa

Land weathering by river runoff makes coastal oceans highly variable ecosystems in terms of seawater pH; however, its effects on biological components and, hence, on the coastal ecosystem functioning has been scarcely addressed. In this study, we determined part of the spatial and seasonal variability of the physical-chemical characteristics of seawater, and life history traits of the neritic copepod Acartia tonsa, along an estuarine-to-coastal zone geographic gradient in the southern Pacific Ocean. There, freshwater influences give rise to sharp gradients in pH, salinity, and temperature, which in turn, may affect the fitness of copepod populations inhabiting along the gradient. In fact, most of the studied copepod traits (egg size, ingestion, and egg production rates) were moderately (r ² = 0.5, p < 0.05) to robustly (r ² = 0.9, p < 0.05) explained by physical-chemical and biological (food abundance and composition) factors. Noteworthy was the negative relationship between low-pH waters and copepod reproductive outcomes. This effect was far evident in the estuarine area where small brood sizes and depleted egg production rates were significantly correlated with low seawater pH (r ² = 0.6, p < 0.05). If short-term episodes of low-pH seawater constitute a significant threshold for reproduction, current findings should stimulate a better description of pH variability in coastal zones, as well as the study of biological consequences derived from the interaction between pH and others drivers on coastal marine populations.