Leaf Stable Isotope and Nutrient Status of Temperate Mangroves As Ecological Indicators to Assess Anthropogenic Activity and Recovery from Eutrophication.

Iana Gritcan,Andrea C Alfaro,Sebastian Leuzinger,Mark Duxbury

doi:10.3389/fpls.2016.01922

Iana Gritcan, Andrea C Alfaro + Show 2 more

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https://doi.org/10.3389/fpls.2016.01922

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Abstract

We measured nitrogen stable isotope values (δ15N), and total phosphorus (%P) and total nitrogen (%N) contents in leaves of the temperate mangrove (Avicennia marina sp. australasica) from three coastal ecosystems exposed to various levels of human impact (Manukau, high; Mangawhai, low; and Waitemata, intermediate) in northern New Zealand. We measured δ15N values around 10‰ in environments where the major terrestrial water inputs are sewage. The highest average total nitrogen contents and δ15N values were found in the Auckland city region (Manukau Harbour) at 2.2%N and 9.9‰, respectively. The lowest values were found in Mangawhai Harbour, situated about 80 km north of Auckland city, at 2.0%N and 5.2‰, respectively. In the Waitemata Harbour, also located in Auckland city but with less exposure to human derived sewage inputs, both parameters were intermediate, at 2.1%N and 6.4‰. Total phosphorus contents did not vary significantly. Additionally, analysis of historical mangrove leaf herbarium samples obtained from the Auckland War Memorial Museum indicated that a reduction in both leaf total nitrogen and δ15N content has occurred over the past 100 years in Auckland’s harbors. Collectively, these results suggest that anthropogenically derived nitrogen has had a significant impact on mangrove nutrient status in Auckland harbors over the last 100 years. The observed decrease in nitrogenous nutrients probably occurred due to sewage system improvements. We suggest that mangrove plant physiological response to nutrient excess could be used as an indicator of long-term eutrophication trends. Monitoring leaf nutrient status in mangroves can be used to assess environmental stress (sewage, eutrophication) on coastal ecosystems heavily impacted by human activities. Moreover, nitrogen and phosphorus leaf contents can be used to assess levels of available nutrients in the surrounding environments.

Highlights

Human activities continue to affect coastal ecosystems throughout the world at alarming rates
For the main sampling year (2015), the highest leaf total nitrogen contents and nitrogen stable isotope (δ15N) values were found at Manukau (2.2 ± 0.1%N, 9.9 ± 0.4, respectively) and the lowest at Mangawhai (2.0 ± 0.1%N, 5.2 ± 0.4, respectively; Figures 1 and 2)
The lowest δ15N value observed at Manukau (7.1 ) was higher than all the δ15N values at Mangawhai, i.e., there was no overlap in δ15N values between the two harbors

Summary

Introduction

Human activities continue to affect coastal ecosystems throughout the world at alarming rates. Habitat destruction (e.g., deforestation, urbanization) and eutrophication (e.g., agricultural runoff, sewage inputs) have been identified as major factors that affect water quality in aquatic ecosystems within coastal areas (Heaton, 1986; Valiela et al, 1992, 1997; Nixon et al, 1996; McClelland and Valiela, 1998; Verhoeven et al, 2006). Excessive nutrient inputs (eutrophication) from intensive agricultural activity and growing populations within coastal regions usually result in rapid degradation of water quality and modifications of ecological features (Schindler, 2006; McGlathery et al, 2007; Heisler et al, 2008; Smith and Schindler, 2009). Aquatic coastal and estuarine primary producers (e.g., phytoplankton, macroalgae, and seagrasses) serve as ‘coastal filters’ and are able to absorb and sequester nutrients into plant biomass (Costanzo et al, 2001; Nixon et al, 2001; McGlathery et al, 2007)

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