Coordinated Observing and Modeling of the West Florida Shelf with Harmful Algal Bloom Application

Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, self‐organizing map, from 23 years (1993–2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry‐derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis : years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep‐ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry‐derived seasonal predictor for major K. brevis blooms is also proposed.

Gag grouper larvae pathways on the West Florida Shelf

Harmful algal blooms of the dinoflagellate Karenia brevis require an upwelling circulation to manifest along the coastline of the West Florida Continental Shelf. Too much upwelling, however, can impede bloom formation by increasing inorganic nutrient levels to the point where faster growing phytoplankton such as diatoms may out-compete the slower growing K. brevis, as occurred in 1998 and 2010. Both 2012 and 2013 experienced persistent upwelling, but only 2012 exhibited a robust harmful algal bloom. Here we examine the subtle differences in the coastal ocean circulation between those two years that led to the disparate bloom evolutions.

Often described as oligotrophic, the west Florida continental shelf supports abundant fisheries, experiences blooms of the harmful alga, Karenia brevis, and exhibits subsurface chlorophyll maxima evident in shipboard and glider surveys. Renewal of inorganic nutrients by the upwelling of deeper ocean water onto the shelf may account for this, but what are the origins and pathways by which such new water may broach the shelf break and advance toward the shoreline? We address these questions via numerical model simulations of pseudo‐Lagrangian, isopycnic water parcel trajectories. Focus is on 2010, when the west Florida shelf was subjected to an anomalously protracted period of upwelling caused by Gulf of Mexico Loop Current interactions with the shelf slope. Origins and pathways are determined by integrating trajectories over successive 45 day intervals, beginning from different locations along the shelf break and at various locations and depths along the shelf slope. Waters upwelling across the shelf break are found to originate from relatively shallow depths along the shelf slope. Even for the anomalous 2010 year, much of this upwelling occurs from about 150 m and above, although waters may broach the shelf break from 300 m depth, particularly in the Florida Panhandle. Such interannual renewal of west Florida shelf waters appears to have profound effects on west Florida shelf ecology.

Material property distributions on continental shelves result from the mixing and modifications of estuarine and deep‐ocean source waters. How this occurs depends on the momentum and buoyancy that are input either locally on the shelf or from the deep‐ocean at the shelf break. We address this question of local versus deep‐ocean forcing for the West Florida Shelf (WFS) using in situ data and a numerical circulation model. The spring and summer seasons of 1998 and 1999 show distinctively different water properties on the shelf and at the shelf break. We account for these differences by a combination of local forcing, independent of the adjacent Gulf of Mexico Loop Current, and interactions of the Loop Current with the shelf. The primary role of the deep ocean is to set the height of material isopleths along the shelf slope. Whether or not these material isopleths broach the shelf break is then a consequence of local, shelf‐wide wind and buoyancy forcing. The subsequent along‐ and across‐shelf distributions are accomplished through a combination of local and deep‐ocean effects, with the bottom Ekman layer being the major conduit for the across‐shelf transport of ecologically important materials.

Blooms of Karenia brevis (Davis) G. Hansen & Ø. Moestrup on the West Florida Shelf: Nutrient sources and potential management strategies based on a multi-year regional study

Blooms of the harmful alga, Karenia brevis on the west Florida continental shelf are thought to initiate offshore before manifesting as a nuisance along the coastline. Contributing to such blooms are a complex sequence of events occurring within oligotrophic waters, which in any given year may or may not be facilitated by the ocean circulation. Once initiation occurs, the delivery from the region of offshore origination to the region of coastline manifestation requires an upwelling circulation, whereby K. brevis cells are advected shoreward along the bottom. The 2018 K. brevis bloom was particularly intense owing to cells from the preceding 2017 bloom being reinforced by a newly formed bloom in 2018, a year when the offshore conditions in spring through early summer were again favorable for bloom development. As an event response to determine the potential for new cells to be delivered to the shore, a glider was deployed from 24 August 2018 to 17 September 2018 with a track line designed to map water properties over the hypothesized initiation region. The coastal ocean circulation during the deployment interval was generally upwelling favorable, but the passage of Tropical Storm Gordon temporarily disrupted this flow, after which K. brevis appeared along the Florida Panhandle coast. Strong upwelling then reestablished and K. brevis was subsequently observed along Florida's east coast. We describe the glider deployment, the K. brevis observations, and we use a numerical circulation model to account for the K. brevis manifestation as occurred along Florida's west, Panhandle, and east coasts.

Ocean-atmosphere heat exchange seasonal cycle on the West Florida Shelf derived from long term moored data

Karenia brevis (K. brevis) blooms regularly in the Gulf of Mexico. However, detection from space still remains a challenge using standard bio-optical retrieval algorithms because of the uncertainties of atmospheric correction and spectral interferences arising in optically complex coastal waters from high concentrations of organic and inorganic materials. We propose a simple red band difference (RBD) technique to detect blooms, coupled with a normalized difference technique to distinguish k. brevis blooms specifically, which we label as the K. brevis bloom index (KBBI). K. brevis blooms studied include those from the Medium Resolution Imaging Spectrometer (MERIS) ocean color measurements off the West Florida Shelf (WFS). As is known, K. brevis has low backscatter characteristics, and as a consequence, it is the chlorophyll fluorescence that dominates the red water-leaving radiance signals (over any elastic scatter). The proposed techniques take advantage of this dominance of the fluorescence signal. With the red water leaving reflectance dominated by fluorescence, which is the case for K. brevis, the reflectance spectra has a local minimum around 665 nm (band 7) and the signal at 681 nm (band 8), which falls in the shoulder of the reflectance peak, has higher values than the signal at band 7 due to the chlorophyll fluorescence emission centered around 685 nm.

Detection of Karenia brevis blooms on the west Florida shelf using in situ backscattering and fluorescence data

Lagrangian circulation and forbidden zone on the West Florida Shelf

Seasonal along-isobath geostrophic flows on the west Florida shelf with application to Karenia brevis red tide blooms in Florida's Big Bend

ABSTRACTSemi‐enclosed seas present complex and nonlinear challenges to understanding climate impacts on fisheries because continental margins restrict poleward movement by taxa seeking to maintain preferred temperatures. The Gulf of Mexico (GoM), a semi‐enclosed, marginal sea surrounded by the North American continent, supports many economically and ecologically important demersal species. A comprehensive assessment of GoM demersal environments is vital to understanding and predicting potential distribution shifts by such taxa. Here we present a Gulf‐wide, interannual spatial evaluation of bottom temperature trends between 1996‐2012. We validated and used bottom temperature products from a regional reanalysis with 24‐h temporal and 1/25° spatial resolution (33,159 grid cells and 6209 days). Ordinary least‐squares and autoregressive models estimated temporal trends and uncertainty, and optimized hot spot analyses identified spatial locales of anomalies. Bottom water cooling occurred along the West Florida Shelf, Florida Keys, northwest Cuba, and the Tamaulipas‐Veracruz Shelf. Warming trends dominated the Bay of Campeche and Louisiana–Texas Shelf. Highest warming and cooling rates were 0.25 (± 0.011)°C year−1 (uncertainty as 95% confidence; located in the Campeche subregion) and −0.12 (± 0.015)°C year−1 (located in the Florida Keys), respectively. Increased duration of Loop Current impingement on the “pressure point” near the Dry Tortugas may drive the observed cooling trends in the eastern GoM, whereas warming trends likely arise from mixing of shallow surface waters. This study highlights the importance of spatiotemporal bottom temperature analyses in complex semi‐enclosed seas where bottom temperature trends may be counterfactual to the long‐term surface warming narrative.

Chapter 4 - Basic Tenets for Coastal Ocean Ecosystems Monitoring

On the remote monitoring of Karenia brevis blooms of the west Florida shelf