A new publication by Dzwonkowski and colleagues from GoMRI funded Consortia CONCORDE and ACER shows that the input of estuarine waters to the Gulf can be important - and not just during high river flow.
It is well known that river dominated estuaries affect the nearshore coastal environment via the discharge or outflowing of river water. The Mississippi River is a good example of this, with the turbidity or suspended sediment plume off the coast of Louisiana being visible in satellite imagery and the freshwater input being detectable by salinity measurements sometimes all the way to the Louisiana-Texas state line.
This has been thought to be especially true for the high discharge or high flow times of the year, such as in the spring. However, the importance of freshwater inputs via the exchange of estuarine and nearshore waters on coastal ecosystem processes under conditions of low river flow is less clear.
A recent publication in Continental Shelf Research led by Dr. Brian Dzwonkowski and others found that even under conditions of low river water input to the nearshore, water column processes – nutrient concentrations, rates of primary productivity, and zooplankton distribution and abundance - were affected by estuarine outflow. This interdisciplinary study was a collaborative effort between the CONCORDE and ACER Gulf of Mexico Research Initiative teams.
Using a unique combination of field sampling, shipboard incubations, imaging systems, monitoring data, and satellite data, Dzwonkowski and colleagues investigated the importance of estuary-offshore interactions during fall (October 2015), typically a period of low river input and well mixed waters. Using salinity data from ship surveys aboard the R/V Point Sur along a transect from the mouth of Mobile Bay to offshore (~55 km in length), they identified the Region of Freshwater Influence - ROFI – and even during low river flow, noted that it extended 30 km offshore. Ocean color data from satellite imagery confirmed this ‘footprint’.
In addition to the salinity data, the ship survey permitted sampling of Gulf water which were analyzed for nutrients (that is the concentrations of the forms of nitrogen, phosphate and silicate) - compounds that allow phytoplankton to grow, as well as chlorophyll (a reflection of the amount of phytoplankton in the water). Samples were taken by ACER’s Dr. Jeffrey Krause for shipboard incubations to measure the amount of total primary production occurring – important as the base of the food chain, along with production for a specific phytoplankton group known as diatoms. Larger plankton size ranges were also observed using the In Situ Ichthyoplankton Imaging System, or ISIIS instrument. The ISIIS unit is towed behind the boat, taking multiple images each second: images are stored and particles in the images are converted, using labor-intensive analyses, to provide data on the abundance and distribution of specific groups.
In the case of this study, groups were diatom chains (a specific group of phytoplankton with glass shells, which create large colonies up to mm or cm in length), Trichodesmium, a group of nitrogen fixing photosynthetic bacteria which also form colonies large enough to be seen by ISIIS, and marine snow, an aggregate of mucus, dead organisms, molts and other flocculent organic material. Corresponding zooplankton samples were taken using traditional net sampling.
Prior understanding of these fall or low discharge periods in the coastal ocean led the scientific team to expect low levels of nutrients and primary production and levels that was not notably different from the shoreline to the farthest extent offshore. However, despite the low levels of riverine water during this time, nutrient concentrations, while still low, were measurably greater in the ROFI. Rates of primary productivity and the abundance of plankton were also significantly higher in the ROFI compared to farther offshore. Interestingly, even though there was low river inflow to the Gulf, it resulted in layering of the water column (a weak stratification) and permitted a distinct assemblage of phytoplankton in the surface waters in the ROFI. Specifically, Trichodesmium cells, which are typically found farther offshore in low nutrient waters, were most abundant in offshore edge of this shallow lens of surface water. The team also found that in this region, much of the primary production was carried out by larger phytoplankton cells. Larger cells are more likely to be eaten directly by the copepods, chateognaths, ostracods and shrimp larvae in the zooplankton community and thus enter the ocean food web.
Serendipitously, the remnants of wind associated with Hurricane Patricia passed over the study area immediately prior to the study. The northern Gulf of Mexico is said to be microtidal meaning that it has small tidal ranges - small differences between high and low tides (on average around 1 foot or 1/3 m). Locals to this area recognize that winds are often more important than the tide in determining how much water we will have to wade through or consider when boating. Dzwonkowski and his team found that the effect of Hurricane Patricia’s winds were to force offshore water back into Mobile Bay prior to the study: water levels at the Dauphin Island monitoring station were the highest recorded since 2012. As the winds weakened and shifted direction, estuarine water flowed out of the bay into the coastal ocean. Salinity and water levels decreased reflecting this flow. Importantly, this outflow was not due to increased river flow entering and flowing through and out the estuary (which would require a longer time period), but simply due to an outpouring of estuarine water from the bay.
A scientific publication contributes to our understanding of how the world works. While oceanographers recognize that physical conditions can, at times, affect chemical and biological conditions, the paper by Dzwonkowski and this team has shown that important ecological processes such as primary production in the coastal ocean can be affected by the outflow of estuarine waters even during low river flow periods. Additionally, this flushing of estuarine waters was not reflective of a period of high river discharge: estuarine input to, and influence on, the coastal ocean may occur in the absence of significant river flow. The authors point out that conditions during their study were similar to those during cold fronts that commonly occur in the northern Gulf during fall, winter and spring. Thus, the influence of estuarine input on nearshore environments, and the resulting biological responses, may be greater that what scientists had understood to be in the past in the northern Gulf.
Data associated with this work are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at http://data.gulfresearchinitiative.org under the following DOIs: 10.7266/N73F4MQ6; 10.7266/N7ZP446V; 10.7266/N7TX3CFP; 10.7266/N79Z930H; 10.7266/N7668B83; 10.7266/N7Q52MQ8; 10.7266/N7K35RRK; 10.7266/N7S46Q0W; 10.7266/N7FQ9TQF; 10.7266/N7XP730K; 10.7266/N78050N9; and 10.7266/N72N509H. In addition, data associated with this manuscript are publicly available from various sources including http://www.mymobilebay.com/, http://oceancolor.gsfc.nasa.gov/cms/, and http://disl.org/.