Consistent Relationships Among Productivity Rate Methods in the NE Subarctic Pacific

Phytoplankton photosynthesize in surface waters, exporting organic carbon to depth through the biological pump. Quantifying productivity and the export of carbon is important to understanding the global carbon cycle and predicting its future changes. An issue in quantifying rates is that the many existing methods are not all equivalent, making comparisons between studies using different methods challenging. Our goal is to compare in situ and in vitro methods in order to identify where methods agree in the NE subarctic Pacific. During this study, we measured productivity using two in situ methods (oxygen/argon ratio and triple oxygen isotope mass balance approaches) and four in vitro methods (C-13, NO3-15, NH4+15, and (H2O)-O-18 uptake rates through incubations), and compared the results with one satellite-based productivity estimate. The in situ carbon export method was consistently higher than the in vitro method, likely due to dissolved organic matter release not included in our incubation measurements. Upwelling bringing low-O-2 water to the surface and the interaction between bloom dynamics and a method's inherent time of integration cause outliers from the relationship. In contrast, in situ and in vitro methods for estimating gross primary production were consistent across a wide range in rates. We find that chlorophyll-a concentration is strongly related to many of our measured rates. Satellite estimates of primary production are consistently different from C-13 incubations. Our identification of consistent trends and causes for disagreement will allow observations from one method to be converted to another, facilitating future comparisons across studies.

Automated summary

Consistent trends between in situ and in vitro methods were observed in estimating gross primary production rates, while significant differences were found in estimating carbon export due to the release of dissolved organic matter not accounted for in the in vitro method. Factors such as upwelling of low-O2 water and the interaction between bloom dynamics and integration time of methods were identified as causes of outliers in the study results.