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You are here: Home › News › Feature Stories › NOAA Scientists Assess Storm Impacts to the Estuarine Food Web
NOAA Scientists Assess Storm Impacts to the Estuarine Food Web
NOAA scientists are vastly improving their understanding of how the Pamlico-Albemarle Sound (PAS) and other large productive estuarine ecosystems recover from stresses imposed by severe storms.
In a study just completed, a team of scientists led by Pat Tester, from NOAA's National Centers for Coastal Ocean Science (NCCOS), examined how the three hurricanes that rocked the Carolinas in late summer of 1999 affected the base of the estuarine food web.
Tester and her colleagues analyzed data on patterns and concentrations of phytoplankton biomass (chlorophyll) throughout the basin before and after the three hurricanes.
Earlier studies had established the baseline phytoplankton dynamics in the Pamlico-Albermarle Sound and its tributaries. In estuarine systems like the PAS and Chesapeake Bay, overall patterns of phytoplankton abundance and distribution are seasonally predictable. The uncharacteristic changes that Tester and her colleagues observed in 1999 were an indication of the magnitude of the disruption from these storms, and also of the ecosystem's ability to return to normal.
Tester and her colleagues observed that, within four months, chlorophyll patterns across the basin had returned to pre-flood levels — though concentrations overall remained higher for about six months after the storms. At the base of the food web, they concluded, the region was remarkably resilient to major storm events, and it recovered quickly.
Tester says the primary trophic level is an excellent indicator of ecological change because it responds rapidly to perturbations. But she cautions against over-interpreting the implications for higher levels in the food web. Responses of higher trophic levels, she says, may vary depending on whether the organisms feed in the water column or are scavengers; how well they can adapt physiologically; and how well they can escape adverse conditions.
These factors may explain why fishing catches were down after the storms even as shrimp harvests were among the highest on record. The following summer (2000) was the best shrimp harvest in a decade and the third highest on record. "What might have been detrimental to the fish was good for the shrimp", Tester says. "They are scavengers. The dead fish were probably more food for them."
The Pamlico-Albemarle basin consists of seven large bodies of water called "sounds." The most prominent features are the Pamlico and Albemarle, four major rivers, and a series of barrier islands. Cape Hatteras, Kitty Hawk, and other tourist destinations along this string of barrier islands off the North Carolina coast draw tens of thousands of visitors each year.
Like other estuaries along the U.S. coast, the region is as an important fish spawning and nursery habitat and food source for migratory birds. Seagrass habitat within the estuarine system exceeds that of the Chesapeake Bay. Nutrient runoff stimulates plant production, which in turn increases production of commercially valuable invertebrates and fish. More than one-half of the U.S. commercial fisheries catch is comprised of estuarine dependent species. In North Carolina, this region accounts for more than 73 percent of the total inshore commercial catch value.
In September-October of 1999, three hurricanes, Dennis, Floyd and Irene, made landfall over a period of about seven weeks. More water flowed into the region than had been previously recorded in any such comparable period. Though the region had experienced periodic hurricanes, three such insults within the same 40 to 50 day period were unprecedented.
The effects on phytoplankton dynamics at the base of the food web were unknown but potentially of great concern given the importance of the region as nursery and spawning grounds. Moreover, by some predictions, this sequence of events could become more common in the future, with hurricanes and severe storms potentially increasing in frequency and severity.
The relationship between ecosystem phytoplankton dynamics and fisheries health is a complex and not fully understood. Tester points to some models that suggest phytoplankton communities where the environment is fluctuating within natural ranges will maintain higher species diversity. This situation favors a diverse food source and a complex food web and forms the basis of a healthy food chain supporting fish populations.
Large pertur-bations such as hurricanes with high nutrient inputs can create sufficient disruption — beyond the normal range of variation — that lead to the dominance of only a few phytoplankton species. A window of opportunity exists in these cases where only a few phytoplankton species dominate: If one or more of those species is a poor food source, the entire food chain can be disrupted.
Hypoxia was also a major concern in the wake of the three hurricanes. At the base of the food web, phytoplankton production depends on nutrient supply, temperature, and light. The onslaught of fresh water to the region from the three storms had created an abundant supply of nutrients. Conditions were ripe for primary production of phytoplankton to explode: The temperature was still high enough to sustain algae growth and there was plenty of light. What would happen when the algae began to die and deplete the waters of oxygen? Nobody knew at that point.
Concerned over potential impacts, Tester started trying to round up the essentials for field sampling as soon as the Hurricane Floyd passed. The situation was still chaotic in the region generally. Many coastal residents had fled the area to escape the hurricane. Most of them could not get back to the coast because of massive flooding, and some were delayed as much as seven days on their return. The campus of East Carolina University had been evacuated because much of it was under water, so scientists there were in no position to sample or evaluate the situation. Many coastal areas were without electricity.
"I went to the local flight school and got an instructor to take me up with a video camera to have a look at the flooded areas and the inlets," says Tester, with the NCCOS Center for Coastal Fisheries and Habitat Research, in Beaufort, N.C. "The plumes of sediment and discolored water were just boiling out of Ocracoke and Oregon Inlets. Clearly, the (usual) one- year flushing rate was going to be shorted to a single month! Estimated flushing rate from the extreme water flow was about 30 days."
Reflecting both a healthy and cooperative working relationship and perhaps also some old-fashioned good luck, a colleague from the Charleston, S.C.- based Coastal Services Center, a partner organization within NOAA, contacted Tester with an offer she couldn't refuse and could hardly have anticipated: availability of the NOAA Twin Otter aircraft, with NASA laser sensors. The two worked out plans for pilots to fly a series of missions over the PAS.
They scheduled the first over-flight for September 25 (the second storm had made land fall on September 16-17.).
In repeated over-flights of the area, the NCCOS scientists measured chlorophyll biomass distribution before and after the storms. Between September 25 and October 31, 1999, NOAA's twin engine research aircraft flew four missions over the PAS and its major tributaries. The scientists flew a fifth mission a year later, after a period of low hurricane activity, to serve as a control since the 1999 hurricane season had begun before sampling measures were in place.
"Now it seems like a small miracle that we were able to sample with three more over flights before October 31", Tester says. "We were dodging frontal systems and another hurricane [Hurricane Irene in mid-October] and trying to negotiate with Cherry Point Marine Air Base to get into restricted air space with the NOAA Twin Otter. Further, we wanted to try and go only on clear days so we could have satellite coverage as well. It turned out that we worked on weekends because that is when the plane could get access to the air space and three of the four dates we had clear weather.
The resulting comprehensive coverage of chlorophyll intensity and dissolved organic matter across the region would have been impossible with surface sampling alone. Airborne and satellite sensors provide a rapid, synoptic view of larger areas than we can easily sample with traditional shipboard methods," Tester explains. "However, it is very important to include surface verification data to validate the data streams from remote detectors."
On each of the over-flight dates, the NCCOS scientists took additional surface samples to help in calibrating data coming from the airborne sensors. They also acquired subsurface data on the degree of water column stratification and light penetration. The NOAA observation aircraft was not available for continued bi-weekly monitoring of the PAS after October 31, 1999 . Tester and her colleagues then substituted data obtained from ocean color satellite images (the SEAWiFS program), making necessary adjustments for comparability. A year later, in October 2000, the research aircraft flew a fifth mission along the same flight path.
On each of these missions, the NOAA research aircraft measured the laser-induced fluorescence of chlorophyll and colored dissolved organic matter (CDOM). The laser system, NASA's Airborne Oceanographic Lidar, transmits a green light pulse into the water, where the light is absorbed by the phytoplankton. A receiver on the aircraft detects the green light reflected from the water's surface and red light emitted by chlorophyll pigment in the algae. At the same time, the SEAWiFS satellite sensors measured changes in water color indicating where concentrations of phytoplankton were located.
Primary production of phytoplankton in the PAS tributaries is closely tied to nitrogen availability. Characteristically, the highest phytoplankton biomass and major blooms occur well upstream from the mouths of these tributaries, at the interface of freshwater and brackish water. The most striking feature of chlorophyll distribution on September 25, 1999 , nine days after Hurricane Floyd had passed through, was the low levels of chlorophyll in the Neuse and Tar-Pamlico Rivers. High river discharge (reaching at one point, 560 times the normal rate) had resulted in a marked reduction of phytoplankton biomass (chlorophyll) in the rivers. Chlorophyll concentrations had been displaced down river into the Pamlico and Albemarle Sounds.
After Floyd, southerly and southwesterly winds helped to distribute the chlorophyll throughout the PAS, though only limited amounts flowed to the continental shelf through inlets in the barrier islands. Increased stream flows that accompanied Hurricane Irene caused a similar displacement of biomass into the PAS that was observed in the fourth and final over-flight of the area for 1999.
As river flows decreased, normal chlorophyll a patterns returned. By January 6, 2000, ocean color imagery indicated higher chlorophyll values in the rivers, returning to a normal pattern. NCCOS scientists continued to take surface samples of chlorophyll a concentrations in the region. Within six months, chlorophyll a concentrations had returned to what they had been before the hurricane season.
According to Tester and her colleagues, two factors likely mitigated the intensity of the resulting phytoplankton blooms and prevented large scale anoxia from occurring: the timing of the third hurricane of the season, Irene, and high concentrations of organic matter displaced from upstream. High winds from Irene oxygenated the water column, the Tester explains, and the high concentrations of CDOM caused less light to penetrate, in effect shortening the water column to less than half of the water depth in some places. Cooling water temperatures, shorter day length, and a lower sun angle slowed the biological decomposition of organic matter and the associated oxygen demand.
The storm surge and flooding from Floyd exceeded the 500-year flood level in some parts of the drainage basin. Material released after extensive and prolonged flooding of marshes and peat-rich areas contributed to the high concentrations of CDOM observed after Floyd. The NCCOS research team observed a highly significant inverse relationship between relative CDOM and chlorophyll a. The large CDOM inputs associated with runoff, they concluded, had lowered the average light penetration into the water column, effectively reducing the productive depth of the water column. This condition reduced the potential for massive blooms that would have used all the available dissolved inorganic nitrogen, and caused widespread anoxia.
The influence of CDOM on productivity, according to the study authors, may represent a major difference between lagoonal estuarine systems such as the PAS and open systems such as the Chesapeake Bay where CDOM can be more rapidly diluted from the system. Most estuaries, like the Chesapeake Bay or San Francisco Bay , have open mouths. They accommodate free exchange of materials from the rivers into the bay and throughout the bay, and free influx of coastal waters on high tide. In a lagoonal estuary, like the Pamlico-Albemarle, barrier islands restrict open flow. Most of the water leaves the PAS by evapotranspiration rather than inlet outflow. Nutrients recycle in this shallow, seasonally warm ìincubator.
While the Pamlico-Albemarle Sound rebounded relatively quickly from the triple assault of the 1999 hurricanes, Tester says it is important to understand that the phytoplankton that returned may not be the same that would have been present had the hurricanes not disrupted. And there's no guarantee the seeming resilience of the Sound's ecosystems can continue to cover from future event-driven perturbations and upsets.
