Decision on bg-2017-528

Dear authors,

Following the reviewers’ suggestions and based to my own reading of your MS and responses to the comments, I believe you paper can be published in Biogeosciences after some significant revisions. The two referees have raised important questions that must carefully addressed in your revised MS. This includes a better description of methodology and the improvement of Nitrogen mass-balance calculations by the different biogeochemical processes you have measured. I also find necessary the addition of “a brief section to the manuscript on the metagenomic analysis of the phytoplankton community” in order to sustain in your MS the discussion on the relationship between N fixation and HABs. I was in general satisfied with most of your answers to the referees comments, and I recommend you incorporate all the necessary information in your revised MS. The two referees also expressed very consistent criticisms on the way you use of two expressions in your MS: the “Nitrogen pendulum” and the “freshwater estuary”. I found these comments very relevant and ask you consider them more seriously.

I think the term “pendulum” must be explained more clearly, as (if I well understood) this is the first time the “Nitrogen pendulum” is described. A pendulum oscillates, swings, up and down and right and left… please be more precise explaining the pertinence of the metaphor and what processes make the pendulum move up and down. The use of this expression in the title of your MS is very abrupt and I don’t think BG readers will understand immediately what you are referring to in the title. I suggest you remove the expression from the title and include it in the abstract together with a short definition of the concept.

I have much more concern with the use of the term “estuary” or “freshwater estuary”, even if the NOAA has defined this last term, the most commonly used definition of an estuary remains the one Cameron and Pritchard (1963): “a semi-enclosed coastal body of water, which has free connection with the open sea, and within which seawater is measurably diluted with freshwater derived from land drainage”. More recently, many studies have extended the definition of the estuary to the “tidal river” (and sometimes “freshwater estuarine zone”), as the upper estuarine region without salt intrusion, but influenced by the tide. In Sandusky Bay there is no salt intrusion and there is no tide. So this ecosystem is very far away from the definition of an “estuary”. I can understand the need for a specific term to describe the mixing zone between a river and a lake as this is an ecosystem very specific properties. “river-lake mixing zone” is probably the most precise term. I can eventually accept “freshwater estuary” in your MS after a careful definition (this definition is in contradiction with that of Cameron and Pritchard), although I feel this expression must be used sparingly and carefully. In all cases the use of “estuary” alone is inappropriate and your results cannot be extrapolated to “estuarine systems” where the salinity gradient is always a major driving force. For instance in estuaries, salinity stress strongly impacts microbial communities; salinity also generates desorption processes that control the availability of phosphate for phytoplankton. As a consequence, P limitation processes as discussed in your MS will not follow classical trends occurring in estuaries. As an example in your abstract the statement “Estuarine systems such as Sandusky Bay are mediators of…” is inappropriate because Sandusky Bay is NOT an estuary. In order to avoid confusion, your MS must be consistent with these definitions; Readers must know immediately that your study concerns FRESHWATERS and not ESTUARINE WATERS.

I will be pleased to read a revised version of your MS, together with a short response to my editorial comments.

With best regards,
Gwenaël Abril, Biogeosciences associate Editor.

Cameron WM and Pritchard DW (1963) Estuaries. In: The Sea, Hill MN (Ed.) Wiley –Interscience, New-York 2: 306-324.