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

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