Using low-cost water presence-absence sensors to assess the regime of headwater streams relevant to the development of aquatic life in the Canary Islands

Abstract

The streams of the Canary Islands are highly vulnerable habitats with the potential to harbour a rich biodiversity, including endemic and endangered species. However, they remain poorly studied because they are commonly perceived as mostly dry and having a low conservation value. This study is the first to characterize the hydrological regime of these water bodies, shedding light on their potential as key habitats for aquatic organisms, as a first step within two projects that study aquatic life in these streams. Temperature and light intensity HOBO Pendant sensors modified for estimating the electrical conductivity of water were installed in 32 remote small stream reaches on the islands of Tenerife, La Palma, and La Gomera. In most of the reaches, one sensor was installed in a riffle and another at the bottom of a pool, in order to distinguish the condition of the reach between the three aquatic phases that control the occurrence of aquatic habitats: flow, disconnected pools and dry riverbed. Some of the sensors could not be recovered or suffered other problems, resulting in 22 reaches with valid records spanning different temporal gradients (from 2 to 12 months). Both relative electrical conductivity and daily variance of temperature were used to determine the daily presence-absence of water. The data served to obtain metrics describing the relative frequencies of the three aquatic phases and report the changes in stream aquatic habitats relevant for aquatic organisms between sampling visits. Eleven reaches showed perennial regimes, four showed regimes dominated by flow sometimes switched to disconnected pools, other four showed regimes rotating between the three phases and the remaining three were dry during all the monitoring periods. These results point to more favourable conditions to the development of aquatic life in these islands than previously expected because, from available data, 50% of the monitored reaches showed continuous flow phase and 73% of them showed permanence of surface water as either flow or disconnected pools phases during >90% of time. These results also demonstrate the value of recording the disconnected pools phase when aquatic life is the focus: if perennial and permanently dry reaches are discarded (eight remaining points), the averaged flow permanence was only 61% of time, while the averaged permanence of surface water increased to 90%. Given the potential of these streams to harbour endemic and endangered species within the context of a global freshwater biodiversity crisis, we call for further efforts to monitor and protect small streams in oceanic islands.