Microplankton Interactions with Decadal-Scale Nutrient Enrichment in a Deep Estuary, with Implications for Eutrophication—Related Ecosystem Stressors

Abstract

AbstractLong-term (decadal) records of microplankton provide insights into how lower trophic levels of coastal ecosystems respond to nutrient enrichment, over and above shorter-term variability. We used a 15-year seasonal census in the Firth of Thames, a deep, nutrient-enriched estuary in northeastern Aotearoa/New Zealand, to determine microplankton responses to enrichment. Kendall trend analyses showed that dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen were enriched by 99% and 34%, respectively, over 15 years, while phosphorus changed little. Larger phytoplankton (> 2 µm) increased by 46%, including 57% increases by diatoms (mainly large centrics with 93% increase) and nanoflagellates (151% increase). Dinoflagellates decreased by 46%, such that the community shifted from dinoflagellate to diatom/nanoflagellate dominance. Within phytoplankton ≤ 2 µm, picoprokaryotes increased by 369%, while picoeukaryotes changed little. Among microheterotrophs (< 200 µm), bacteria increased by 89%, and small oligotrichs increased by 53%. Trend analyses and multivariate general additive modelling showed that microplankton biomass responded primarily to increased DIN over the 15-year period and secondarily to stratification variation at shorter time scales. The changed biomasses and community composition are explained as responses to increased N:P and food-web interactions. Deleterious changes included increased toxic Pseudo-nitzschia abundance and potentially reduced nutritional quality of the phytoplankton community for grazers. The increased N and larger diatoms indicated potential for increased deposition to sediments, possibly explaining previous observations of lowered denitrification in the Firth during the time series period. The results indicated a continuation of enrichment the Firth has received over decades, with implications for expression of ecosystem stressors of acidification and hypoxia.