THE NEEDED FOCUS OF A GULF HYPOXIA REASSESSMENT

1.         Efficient and effective solutions to the issue of hypoxia in the Gulf of Mexico need to be based on sound scientific data and information.

2.         There is a simple question that policy makers and resource managers need clear answers to as a basis for establishing efficient and effective hypoxia control strategies.

3.         This question has not been clearly posed and answered, either in the CENR hypoxia assessment reports, the scientific literature, or the recent reviews of USEPA’s Region 4 whitepaper* .

4.         The question is:

“Which forms of which nutrients need to be controlled by how much, from which sources, and at what times of year to reduce the size of the hypoxic zone.”

5.         Because various forms of, for example, nitrogen and phosphorus are related in the formation of hypoxia, and hypoxia dynamics vary over time, failure to provide an unbiased, accurate, and reasonably precise answer to this question is likely to lead to inefficient, ineffective, and costly control strategies, even under an adaptive management scheme.

6.         Below, I provide some extracts from the recent reviewers comments on the USEPA Region 4 reports to illustrate the ongoing diversity of views among experts on the subject of nutrient control to reduce hypoxia.

7.         USEPA agrees that “These issues need to be addressed to clarify and definitively determine the mechanistic linkages between nutrient supply, productivity and excessive oxygen consumption (hypoxia) in the Gulf.”

8.         The planned 2007 hypoxia reassessment needs to focus on providing an unbiased, accurate, and reasonably precise answer to the above question, which may or may not be derived by consensus. It will be essential to structure the reassessment carefully, including the setting of an appropriate scientific framework, the selection of scientific experts, and the selection of objective reviewers.

*COMMENTS, SUMMARY REPORT, EXTERNAL PEER REVIEW OF THE ROLE OF NITROGEN/PHOSPHORUS IN CAUSING OR CONTRIBUTING TO HYPOXIA IN THE NORTHERN GULF (2005).

Illustration of the diversity of views among experts on the subject of nutrient control, as expressed in exerts from the External Review Comments on the USEPA Region 4 papers (emphasis added).

[Note: the reviewers’ comments on the USEPA Region 4 reports need to be considered in the context of the approved 2001 hypoxia Action Plan, which accepts that hypoxia in the Gulf is “driven primarily by excess nitrogen” and that a 20-40% reduction in average annual total nitrogen is needed to reduce the size of the hypoxic zone. The Action Plan expects that the nitrogen control actions also will reduce phosphorus in the interior basins (not in the Gulf).]

To quote Reviewer Number 8:

...given the large amount of information in the CENR Reports and literature, how did we end up

 with a Hypoxia Action Plan based on management of the single nutrient, nitrogen?”

The Rabalais et al. (1999) report does argue that nitrogen is the limiting nutrient for overall productivity, and I would submit that this is inference and not entirely supported by the data presented.

Reviewer 1

I wholeheartedly agree with the concept of a balanced approach to managing both nitrogen (N) and phosphorus (P) ..

I believe information has been presented which shows that nutrient loading to the northern Gulf from the Mississippi and Atchafalaya River Basins varies seasonally. Thus, reduction goals and strategies to achieve them should be seasonally flexible to reflect major shift in P or N loadings.

Reviewer 2

Studies have indicated that offshore waters of the GOMx are nitrogen limited whereas near shore waters may be phosphorus limited. However, the (straight-forward?) cause and effect relationship between these variables and phytoplankton biomass and (or) growth rate (as it may or may not relate to Redfield requirements) is difficult to develop.

...physiologically- and taxonomically-distinct phytoplankton responses.

Reviewer 3

By all accounts, phytoplankton production is severely P limited, yet enigmatically there is high phytoplankton biomass and productivity, days to weeks downcurrent from the relatively phosphorus-rich river plume. With such strong P limitation, how can this be? I offer the following interpretation ..

Although the above interpretation is admittedly speculative ...

Stay the course on implementing the N reduction goal.

 ..it is premature to add P goals without much more careful analysis.

Simpler, empirical models suggest that loadings of nitrate would have to be reduced from 30 to 45% in order to meet the Action Plan goal for attenuation of hypoxia (Scavia et al, 2004). At this point, it really doesn’t matter where in that range the true target lies. ..the models and targets can be continually refined, in an adaptive management context.

Reviewer 4

What might be controversial is the degree to which N and P contribute to hypoxia, ...

Reviewer 5

...the report correctly points out that questions should be raised about justifying N as the only limiting nutrient of concern ...

 ...previous nutrient concentration and input ratios published by Turner and Rabalais 1991, Justic et al. 1995, and Rabalais et al. 1999 were shown as DIN:Total P, as opposed to the more commonly used “conventional” ratio of DIN:DIP (dissolved N compounds over dissolved P compounds) (P. 1 of report). When it comes to biological reactivity, it makes more sense to consider the ratio of dissolved forms of one nutrient to another (dissolved species tend to be most readily available forms for uptake and growth) as opposed to comparing dissolved forms of N to total (dissolved and particulate) forms of P.

To what extent nutrient (specifically N) loading has increased, such that in some areas of MARDA N uptake/utilization may be periodically saturated (Fig. 17, Jan. 2004 report) and some other factor (P, light, or both) is limiting remains unclear. These bioassays should be conducted under in situ light and temperature conditions, using the naturally-occurring phytoplankton community as test organisms.

Furthermore, as N (or P) inputs are reduced and productivity is altered, the relative amounts and importance of internal cycling of these nutrients are likely to change as well. There is no assurance that these cascading events will occur in a linear or (at this point in time) predictable fashion. The various versions of the report refer to this important aspect of MARDA and N. GOM nutrient dynamics. It is premature to make recommendations pertaining to amounts and types of nutrient reductions needed to control production and hypoxia, given the lack of information on the relative roles and importance of internal N and P cycling.

...there is a great deal of uncertainty as to how, when, where and in what quantities “new” production is transported to the zones supporting hypoxic bottom waters.

...it remains unclear where, when and how long periods of maximum primary production and phytoplankton biomass formation persists in the GOM (P. 12 of the report). This is a highly relevant and critical informational need, essential to knowing whether maximum productivity coexists with N or P limitation (or co-limitation), and whether the fate of nutrient limited productivity can spatially and temporally be linked to hypoxia dynamics.

It is less clear where the zone of maximum productivity exists during this period.

With respect to the abovementioned topics, critical questions, that previous work or the EPA reports do not clarify, should be addressed. These include:

1.          Where is the zone of primary production and phytoplankton biomass accumulation in the MARDA?

2.          What is the importance of this “new” production relative to allochthonous organic matter (including terrestrial and riverine produced organic C sources)?

3.          The seasonal patterns of oxygen consumption and hypoxia formation are important. This information should be combined with question 1 in order to develop some predictive relationship(s) between nutrient loading and hypoxia formation/dynamics.

4.          Is “excess” N really delivered to the coastal-shelf marine environment?

5.          A physical-biological integrated model needs to be developed that can predict hypoxic volume, extent and duration interactive with and independent of nutrient enhanced primary production. Is such a model available?

6.          What is the relative importance of autochthonously-produced vs. allochthonously supplied organic C in oxygen consumption and hypoxia dynamics of the GOM?

7.          What is the temporal and spatial lag between nutrient-enhanced “new” production and hypoxia dynamics in the plume and GOM receiving waters?

8.          To what extent can and should we solely rely on Redfield ratios to determine (and confirm) that either N or P or both are limiting Ppr. Parallel confirmation (of nutrient limitation and nutrient-productivity interactions) based on nutrient addition bioassays with natural phytoplankton communities under ambient light/temperature and grazing conditions, is needed as a more direct line of evidence for specific types of nutrient limitation and phytoplankton growth responses.

Reviewer 6

P may be a very important limiting nutrient in this system and have a major role in controlling hypoxia, but the evidence given in this report is just a first step in the process.

There is little in this report suggestive of nitrogen (N) limitation, nonetheless, N limitation cannot be ruled out, ...

The Redfield ratio as used by Rabalais et al. (1999) is unusual since in the river water it refers to DIN:TP, while in the Gulf it refers to DIN:DIP, the more conventional usage. I am not sure why the river data was treated this way, perhaps due to data availability, but the EPA report is correct to point out the unusual application of this ratio and the possibility of confusion.

Reviewer 7

In my professional opinion, the higher values that occurred from January through May are indeed likely to suggest phosphorus limitation along this sampling transect. On the other hand, the lower ratios between June and December are too close to the Redfield ratio to be interpreted in this manner, without much more information on the relative rates of uptake and recyling of these elements.

...there is a large body of evidence out there on both phosphorus and nitrogen limitation ...

Reviewer 8

The documents raise serious concerns about a management strategy focusing solely on a single nutrient element. I contend that given the complex nature of the problem and limited understanding of many issues, only further monitoring coupled with an adaptive management approach will resolve some of the questions raised. Such a management approach should entail an extensive ongoing program of monitoring both river and shelf water properties, combined with a comprehensive program of modeling that will both enhance understanding of mechanisms and allow for prediction of outcomes in response to different management actions.

Action must be taken to reduce BOTH nitrogen and phosphorus in receiving waters of the Mississippi River.

Finally, further study is needed to better understand linkages between nutrient inputs, algal productivity, carbon fluxes to bottom waters, and hypoxia.

...given the large amount of information in the CENR Reports and literature, how did we end up with a Hypoxia Action Plan based on management of the single nutrient, nitrogen? To quote from the Rabalais et al. (1999) CENR report:

“Managing for a single nutrient is difficult for the large Mississippi River system because N and P may change together but not linearly, and Si is complexly interrelated with P dynamics and/or water retention in the watershed.”

Similarly, from Brezonik et al. (1999):

“Differences in results [increases in average dissolved oxygen concentrations] between reductions in N and P loadings were generally not significant…”

Without understanding linkages of where and when organic matter fueling hypoxia is produced, it is not feasible to discount either nitrogen or phosphorus as critical limiting nutrients.

The Rabalais et al. (1999) report does argue that nitrogen is the limiting nutrient for overall productivity, and I would submit that this is inference and not entirely supported by the data presented.

...algal biochemical ratios of N:P, and ratios of uptake, can change depending on nutrient state ... and differ substantially among taxa...

...the document makes a valid criticism of the somewhat arbitrary use of the DIN:TP ratio.

...the actual timing in supply of organic matter fueling oxygen consumption is not well known.

Reviewer 9

The fundamental recommendation to reduce nitrogen (N) and phosphorus (P) loading to the Gulf of Mexico “there may be a considerable benefit to reducing both nutrients in order to restore water quality” that is reached in this document is sound.

The data here do not distinguish between point source and non-point source origins of P loading to the Gulf of Mexico. A much more complete accounting would be necessary to accomplish this.

A better estimate of limitation would come from the loading of total N and total P to the Northern Gulf, and then adjusting for differential losses of the two elements.