Negative Effects of Diurnal Changes in Acidification and Hypoxia on Early-Life Stage Estuarine Fishes

Brooke K Morrell,Christopher J Gobler

doi:10.3390/d12010025

Abstract

Estuaries serve as important nursery habitats for various species of early-life stage fish, but can experience cooccurring acidification and hypoxia that can vary diurnally in intensity. This study examines the effects of acidification (pH 7.2–7.4) and hypoxia (dissolved oxygen (DO) ~ 2–4 mg L−1) as individual and combined stressors on four fitness metrics for three species of forage fish endemic to the U.S. East Coast: Menidia menidia, Menidia beryllina, and Cyprinodon variegatus. Additionally, the impacts of various durations of exposure to these two stressors was also assessed to explore the sensitivity threshold for larval fishes under environmentally-representative conditions. C. variegatus was resistant to chronic low pH, while M. menidia and M. beryllina experienced significantly reduced survival and hatch time, respectively. Exposure to hypoxia resulted in reduced hatch success of both Menidia species, as well as diminished survival of M. beryllina larvae. Diurnal exposure to low pH and low DO for 4 or 8 h did not alter survival of M. beryllina, although 8 or 12 h of daily exposure through the 10 days posthatch significantly depressed larval size. In contrast, M. menidia experienced significant declines in survival for all intervals of diel cycling hypoxia and acidification (4–12 h). Exposure to 12-h diurnal hypoxia generally elicited negative effects equal to, or of greater severity, than chronic exposure to low DO at the same levels despite significantly higher mean DO exposure concentrations. This evidences a substantial biological cost to adapting to changing DO levels, and implicates diurnal cycling of DO as a significant threat to fish larvae in estuaries. Larval responses to hypoxia, and to a lesser extent acidification, in this study on both continuous and diurnal timescales indicate that estuarine conditions throughout the spawning and postspawn periods could adversely affect stocks of these fish, with diverse implications for the remainder of the food web.

Highlights

Estuaries, rivers, and coastal bays function as essential spawning grounds and nursery habitats for forage fishes, which are pivotal components of marine food webs [1]
This study revealed the ability of diurnal fluctuations in pH and dissolved oxygen (DO) to significantly reduce the size, hatch time, hatch success, and larval survival of two of three species of forage fish studied here
Estuaries serve as crucial spawning and nursery grounds for forage fish, and can regularly experience diurnal fluctuations in low pH and low DO, a potential threat to fish success and survival

Summary

Introduction

Rivers, and coastal bays function as essential spawning grounds and nursery habitats for forage fishes, which are pivotal components of marine food webs [1]. Large declines in early-life stage survival can translate into significant decreases in recruitment and an abundance of fish stocks [4]. The phenomena of ocean acidification (decreased pH) and hypoxia (low dissolved oxygen) associated with global climate change have been increasingly prevalent in world oceans in recent decades [5,6]. These stressors add another layer of complexity to the physiological challenges for early-life stage fishes [2,7,8]. Acidification and hypoxia can be especially acute in coastal ecosystems where unfavorable low pH and low DO conditions are intensified by eutrophication [9,10]

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