Effects of the Wairarapa and Hikurangi Eddies on Chlorophyll Fluorescence off the East Coast of New Zealand

Mesoscale eddies (70-160 km diameter) are ubiquitous, ephemeral features throughout global oceans that drive biogeochemical cycling and ecosystem distribution. The Wairarapa (WE) and Hikurangi (HE) Eddies are semipermanent, anticyclonic (counterclockwise, downwelling) eddies off the east coast of New Zealand that have previously been examined primarily by satellite imagery to determine their location, persistence, and effect on near surface chlorophyll-a relative fluorescence. They are reinforced by eddies shed from the East Cape Eddy to the north which is driven by the micronutrient-rich East Auckland/East Cape Current rounding the East Cape of the North Island. This study used in-situ methods from annual cruises conducted in the austral summer to analyze the effects of eddy current motion on chlorophyll both at the surface and through the water column. The eddies were identified using Acoustic Doppler Current Profiler data. Surface chlorophyll-a relative fluorescence was measured using a flow-through system. Chlorophyll-a relative fluorescence at depth was collected via conductivity-temperature-depth casts. The WE was identified in every cruise examined, but the HE was only identified in 2 of 4 cruises and was overwhelmed in the other cruises by the d’Urville Current flowing out of Cook Strait. Both eddies had significantly higher chlorophyll than the offshore Bay of Plenty control region, indicating the incorporation of other, macronutrient rich water, likely from the d’Urville and Wairarapa Coastal Currents. The WE had a concentrated, intensified deep chlorophyll maximum potentially due to downwelling concentrating phytoplankton, indicating the need for further investigation of eddy effects on chlorophyll beyond the surface. The HE had increased surface chlorophyll in the center in spite of simplistic models suggesting decreased chlorophyll in the center of anticyclonic eddies