# Presentation: 344666

## Subgroup 1 Charge:

- A. Address the state-of-the-science and the importance of various processes in the formation of hypoxia in the Gulf of Mexico. These issues include:
- 	i. increased volume or funneling of fresh water discharges
- 	ii. changes in hydrologic or geomorphic processes
- 	iii. increased nutrient loads due to coastal wetlands losses, upwelling or increased loadings from the Basin
- 	iv. increased stratification, and seasonal changes in magnitude and spatial distribution of stratification and nutrient concentrations in the Gulf;
- 	v. temporal and spatial changes in nutrient limitation, as significant factors in hypoxic zone formation;
- 	vi. the implications of reduction of phosphorus or nitrogen without concomitant reduction of the other.
- B. Comment on the state of the science for characterizing the onset, volume, extent and duration of the hypoxic zone.

## Physical context:

- Plume-induced stratification a strong control over hypoxia, contributes to regional circulation.
- Stratification near bottom/loss of O2 at this interface also important process.
- Duration quite variable, driven by shifts in plume and changes in stratification.

- Rowe and Chapman (2002):  Three different regions to consider; physical and biogeochemical processes are different in each.  Physical factors significant in determining hypoxia duration, extent

## Circulation:

- Regional circulation driven by Loop Current and its Eddy System, but local processes, particularly wind, more important in shallower regions.
- Bird’s foot blocks along-isobath flow, creates two cells.
- Smith and Jacobs (2005): spring/summer flows weak, variable, generally W to E.
- Zavula et al. (2003): E to W near-coast, except July.
- Inshore of 50 m isobath, strongly wind/weather driven.

## Still needed:

- Tidal dissipation and its role
- Status and validity of physical circulation and mixing models

## Carbon loads

- Carbon inputs:  both terrestrial and autochthonous; variable estimates of burial, reactivity.
- Mobile muds important in moving C on shelf and beyond.  Some C utilized in this layer; some buried, some removed from shelf.
- Changes (decreases) in sediment loads, wetlands losses over past 50 years.  Disagreement in amount of OC from wetland loss—from an insignificant C source to a highly significant one.
- Ms. River major source of DOC to shelf; also significant in situ production
- Terrestrial, sediment, and autochthonous inputs necessary to drive current hypoxic extents

## Still needed:

- Better understand role of C from eroding wetlands, other terrestrial sources: relative lability, % utilization, transport, burial?
- Consensus re: relative loads of terrestrial OC vs. autochthonous OC
- Are the differences in OC degradation in the western region of the hypoxic zone (relative to eastern region) important to management questions – or focus on the near-plume effects where hypoxia occurs most frequently?

## Nutrient inputs and cycling

- Substantial inputs of N and P from Basin enhance primary production; spatially and temporally variable.  Also significant loads from offshore, atmospheric deposition.
- Variability in loads and transparency modulate the production of C, and its fate in these three zones on the shelf
- Role of denitrification?  Significant relative to P burial?
- Role of wetlands in altering nutrient loads under debate
- Substantial fraction of P is particle bound.  What is its fate?  Low bioavailability in freshwater, but desorbed and available on the shelf

## Historical evidence for hypoxia

- Relatively few studies, but proxies consistent with hypoxia emerged in the mid 20th century and continued through the end of the century
- 		pigments characteristic of anoxygenic spp
- 		changes in the foram community
- 		preservation of algal pigments, other 	biochemical surrogates
- Pattern of change is consistent with increased nutrient loading from the Mississippi River Basin

## N vs P

- Molar ratios vary between 5 and 300; also flushing rates, light, turbidity, recycling rates all influence nutrient uptake and limitation; Rowe and Chapman zones likely different in times and extent of N and P limitation, with inshore areas strongly P limited during important spring/summer period, and offshore N limited.
- Still in question:  how these zones differ in their contributions to extent of hypoxia.
- What is the linkage between high primary production inshore and hypoxia located further offshore?

## Role of Si

- Increases in N and P have altered N:Si, P:Si ratios.  Strong potential for shifts in phytoplankton species composition, and likely shifts in C utilization by higher trophic levels.
- Questions about how these shifts may alter resource species, also their inputs/impacts on hypoxia

## Reduce N and/or P?

- N fluxes from Basin have decreased in recent updates, but P fluxes have increased
- Evidence growing that both N and P are important nutrients for control.  Suggests that dual nutrient control strategies are warranted
- Current Action Plan calls for N reductions, but explicitly states many processes under way will also remove P
- Many of the programs underway (forest buffers, 319 projects) will actively reduce N and P
- No rationale for P reduction alone.

## Subgroup 1 Charge:

- A. Address the state-of-the-science and the importance of various processes in the formation of hypoxia in the Gulf of Mexico. These issues include:
- 	i. increased volume or funneling of fresh water discharges
- 	ii. changes in hydrologic or geomorphic processes
- 	iii. increased nutrient loads due to coastal wetlands losses, upwelling or increased loadings from the Basin
- 	iv. increased stratification, and seasonal changes in magnitude and spatial distribution of stratification and nutrient concentrations in the Gulf;
- 	v. temporal and spatial changes in nutrient limitation, as significant factors in hypoxic zone formation;
- 	vi. the implications of reduction of phosphorus or nitrogen without concomitant reduction of the other.
- B. Comment on the state of the science for characterizing the onset, volume, extent and duration of the hypoxic zone.