Effect of prolonged seawater warming on Zostera marina ecotypes of the northeast Pacific

Abstract

Seagrass meadows are complex, multi-species ecosystems that are facing increasing seawater temperatures with climate change. Previous experimental studies have imposed relatively short-term elevated seawater temperatures for understanding stress tolerance and have revealed a disparate range of responses by the seagrasses themselves, as well as by organisms that inhabit seagrass meadows. Variation in response may be due to differences in environmental context, population-level characteristics, temperature regime, and metrics investigated. To tease apart environmental versus intrinsic population factors, we conducted a ‘common garden’ experiment with eelgrass (Zostera marina L.) from four populations of the greater Puget Sound region (Washington, USA) to test effects of slightly elevated seawater temperatures (+2.5 °C or + 5.6 °C compared to ambient) over long durations (1–2 years), to emulate shifts in mean temperature as predicted for climate change. We measured a comprehensive suite of demographic and morphological traits to be inclusive of traits measured amongst other studies, as well as wasting disease and epiphyte characteristics. We found that, under elevated temperatures, Z. marina exhibited enhanced response or trade-offs in demographic and morphological characteristics depending on season, magnitude of temperature increase, duration of elevated temperature, and donor population. Under +2.5 °C conditions, Z. marina exhibited primarily enhanced responses, such as production and maintenance of longer leaves year-round, and more, wider leaves and increased shoot production during spring. Under +5.6 °C conditions, aboveground morphological responses were even more enhanced and lasted longer, but with trade-offs with shoot production and maintenance of leaf tissue during winter months, and only in the first year. By the second year of exposure to +5.6 °C conditions, plants were not able to maintain increased shoot production rates. These results suggest Z. marina response to slightly elevated temperatures may be modulated by seasonal light availability, so enhanced responses may be only temporary and could precede longer-term decline within a small margin of temperature increase if conditions persist >1 year. Additionally, treatment-by-site interactions did occur, under higher elevated temperature conditions, suggesting resilience may vary by ecotype. Lastly, plants in heated treatments had fewer epiphytes and exhibited seasonal differences in eelgrass wasting disease, suggesting seawater warming could impact eelgrass meadows as whole ecosystems. Further research is needed to understand how these responses may differ across latitudinal gradients, temperature regimes, and multi-species assemblages, and also implications for carbon cycling and habitat provisioning ecosystem functions of eelgrass meadows.