The stable isotope hydrology of Sable Island, Nova Scotia, Canada with implications for evaluating the water budget of wild horses.

Overview

• This study analyzed the stable hydrogen and oxygen isotopes in various water sources and feral horse hair on Sable Island, Nova Scotia, over five years to understand the island’s water cycle and its implications for wild horse water use.

Study Purpose and Context

• Investigate the stable isotope hydrology of Sable Island to better understand the water budget, especially related to wild horses.
• Use isotopic measurements as tracers to distinguish between different water sources and processes.
• Examine how these isotopic signatures vary seasonally and spatially across water reservoirs and biological samples (horsehair and plants).

Methods and Data Collection

• Sampling period: September 2017 to August 2022 (five years).
• Collected integrated monthly precipitation samples to measure hydrogen (H) and oxygen (O) isotope ratios.
• Obtained samples from deep groundwater, shallow groundwater, surface waters, fog, plant leaf water, and horsehair from feral horses.
• Measured isotopic values expressed in delta notation relative to standard references, in per mil (‰).

Key Findings: Isotopic Signatures in Water Sources

• Precipitation:
  • Hydrogen isotopes (δH) ranged between -66‰ and -15‰.
  • Oxygen isotopes (δO) ranged between -9.7‰ and -1.9‰.
  • Seasonality of isotope values was weak, showing limited variation month to month.
  • Developed a local meteoric water line (LMWL): δH = 7.22 (±0.21) × δO + 7.50 (±1.22)‰, characterizing the relation between δH and δO in local precipitation.
  • Annual, amount-weighted average isotope values were -36‰ (H) and -6.1‰ (O), with some variability.
• Groundwater:
  • Deep groundwater exhibited more negative δH and δO values than average annual precipitation. This implies recharge mainly occurs during winter precipitation, which typically is isotopically lighter.
  • Shallow groundwater showed isotope values consistent with mean precipitation or a mixture of precipitation and local seawater influence.
• Surface waters:
  • Had more positive δH and δO values compared to groundwater, indicating relatively isolated water bodies that do not mix extensively with groundwater.
• Fog:
  • Had significantly more positive δH and δO values than local precipitation but similar oxygen-excess (O-excess) values, suggesting different formation or evaporation processes.

Plant Water and Horse Hair Isotope Insights

• Plant Leaf Water:
  • Isotopic composition indicates that plants primarily use groundwater as their water source, linking surface ecosystem directly to groundwater rather than precipitation or fog alone.
• Horsehair:
  • Sampled hairs from 4 wild horses demonstrated isotopic variations similar to precipitation patterns on the island but without clear seasonality.
  • Significant differences in mean δH among individuals suggest that horses within different areas access or metabolize water differently, reflecting spatial variation in water use or sources.

Implications and Contribution

• The study provides a foundational isotope data framework for understanding the hydrology of Sable Island and its wildlife.
• Demonstrates that isotope measurements can help elucidate water source usage by plants and feral horses, critical for assessing their ecology and survival in this unique environment.
• Improves understanding of recharge timing and water mixing processes on the island, which is important for conservation and management of water resources and feral horse populations.
• Supports ongoing studies using isotopes to monitor animal movement, diet, and habitat use related to water availability.