Teleconnections, Local Drivers, and Long-Term Changes in Nitrate Dynamics in an Inland Sea

The supply of nutrients determines coastal productivity, yet long-term measurements of nutrients coincident with other physical and biological variables are relatively rare. While the role of large-scale processes such as upwelling on productivity are understood on relatively linear shores, the importance of these versus local processes in highly reticulate inner coastal shores is less studied. Here we report a long-term (22 year) data set on seawater nitrate collected adjacent to the Seattle Aquarium in Elliott Bay in the central Puget Sound region. Approximately weekly seawater collections and seawater features including temperature and dissolved oxygen, showed strong seasonal cycles, with the lowest nitrate concentrations in late summer intervals in August and September. Nitrate seasonal cycles were disrupted in 2015, coincident with a major marine heatwave in the northeast Pacific Ocean, when longer cycles appear to be emerging. The shared periodicities between nitrate concentrations and large scale oceanographic indices including the PDO, NPGO, and ONI have increased through time at 2 and 4 years. Measurements of chlorophyll a and zooplankton biomass collected over some of the span of the nitrate data indicated that both lag seasonal nitrate cycles. Long-term measurements also indicated declining chlorophyll concentrations, while seawater temperatures had an upward trend. Increasing sea surface temperature was confirmed for the region at a different site at Race Rocks on the Canadian coast. This central region of Puget Sound shows evidence of non-stationary dynamics and long-term change in the availability of nitrate.