Did the northeastern Gulf of Mexico become greener after the ...

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L09601, doi:10.1029/2011GL047184, 2011

Did the northeastern Gulf of Mexico become greener after the Deepwater Horizon oil spill? Chuanmin Hu,1 Robert H. Weisberg,1 Yonggang Liu,1 Lianyuan Zheng,1 Kendra L. Daly,1 David C. English,1 Jun Zhao,1 and Gabriel A. Vargo1 Received 17 February 2011; revised 28 March 2011; accepted 30 March 2011; published 3 May 2011.

[1] Assessment of direct and indirect impacts of oil and dispersants on the marine ecosystem in the northeastern Gulf of Mexico (NEGOM) from the Deepwater Horizon oil spill (April – July 2010) requires sustained observations over multiple years. Here, using satellite measurements, numerical circulation models, and other environmental data, we present some initial results on observed biological changes at the base of the food web. MODIS fluorescence line height (FLH, a proxy for phytoplankton biomass) shows two interesting anomalies. The first is statistically significant (>1 mg m−3 of chlorophyll‐a anomaly), in an area exceeding 11,000 km 2 in the NEGOM during August 2010, about 3 weeks after the oil well was capped. FLH values in this area are higher (i.e., water is greener) than in any August since 2002, and higher than ever since 2002 in an area of ∼3,000 km2. Analyses of ocean circulation and other environmental data suggest that this anomaly may be attributed to the oil spill. The second is a spatially coherent FLH anomaly during December 2010 and January 2011, extending from Mobile Bay to the Florida Keys (mainly between 30 and 100‐m isobaths). This anomaly appears to have resulted from unusually strong upwelling and mixing events during late fall. Available data are insufficient to support or reject a hypothesis that the subsurface oil may have contributed to the enhanced biomass during December 2010 and January 2011. Citation: Hu, C., R. H. Weisberg, Y. Liu, L. Zheng, K. L. Daly, D. C. English, J. Zhao, and G. A. Vargo (2011), Did the northeastern Gulf of Mexico become greener after the Deepwater Horizon oil spill?, Geophys. Res. Lett., 38, L09601, doi:10.1029/2011GL047184.

1. Introduction [2] A massive explosion occurred on the Deepwater Horizon (DWH) oil drilling platform in the northeastern Gulf of Mexico (NEGOM) on 20 April 2010, after which the platform sank to the 1500‐m seafloor on 22 April 2010. This tragic event cost 11 lives and led to the largest offshore oil spill in U.S. history. It has been estimated that about 4.9 million barrels of crude oil and an unknown amount of gas were released into the ocean over 83 days (22 April – 14 July; McNutt et al., 2011). [3] The oil spill, plus the application of chemical dispersants, poses an unprecedented threat to the GOM, its

1 College of Marine Science, University of South Florida, St. Petersburg, Florida, USA.

Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2011GL047184

coastline, sensitive ecosystems, and economy. Over the past several decades, there have been a number of field and laboratory studies investigating the ecological impacts of oil spills in the aquatic environment [Teal and Howarth, 1984]. Of these, only a handful of studies focused on phytoplankton (base of the food web), and results varied substantially. Some studies observed an increase in phytoplankton growth [Parsons et al., 1976; Linden et al., 1979; Vargo et al., 1982], while others found inhibition of photosynthesis [Nuzzi, 1973; Dunstan et al., 1975; Miller et al., 1978]. In some cases, increased biomass was attributed to the reduction in zooplankton [Miller et al., 1978; Linden et al., 1979]. Batten et al. [1998] found no significant oil spill impact on plankton in the Southern Irish Sea. In contrast, a phytoplankton bloom was observed following the IXTOC‐1 oil spill in the southern GOM during 1979 [Jernelöv and Lindén, 1981]. The use of chemical dispersants adds another complexity, as the short‐ and long‐term effects of dispersants on phytoplankton abundance and community structure rarely have been studied [e.g., Brown et al., 1990]. [4] The NEGOM is a complex marine environment, where coastal runoff from point (e.g., Mississippi River) and non‐point sources, upwelling of deep ocean water onto the continental shelf, tele‐connections by the Loop Current and its eddies, tropical storms, and their concomitant effect on marine organisms result in a dynamic marine ecosystem. Thus, despite efforts by federal and state agencies, academic institutions, environmental groups, and private sector entities, it is challenging to assess the potential impacts of the DWH oil spill on local and Gulf‐wide ecosystems, especially when “baseline” information on the natural variability of relevant environmental and biological variables is largely unknown [National Research Council, 2003]. One possible exception is phytoplankton biomass, as modern satellites are expected to provide consistent and synoptic estimates of several bio‐ optical properties of the surface ocean over multiple years. [5] Here, using MODIS satellite observations between July 2002 and January 2011, circulation models, and other data, we attempt to address two questions: (1) Was there a significant change in surface phytoplankton biomass in the NEGOM after the oil spill and (2) if so, was the change related to the spill?

2. Data Selection and Processing Methods [6] Although standard data products of MODIS chlorophyll‐a concentrations (Chl in mg m−3) were available, their accuracy and consistency are questionable for the NEGOM [Hu et al., 2003a; D’Sa et al., 2006], mainly because of the interference of variable colored dissolved organic matter (CDOM) with the chlorophyll algorithm, as sediment

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HU ET AL.: GULF OF MEXICO GREENER AFTER OIL SPILL?

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Figure 1. (a) Frequency of surface oil appearance in satellite imagery (number of valid observations = 58). The oil rig location is marked with “X”, and the dashed rectangular box shows the NEGOM region (27.5°N to 31°N, 91°W to 82.6°W). (b) MODIS FLH anomaly (mW cm−2 mm−1 sr−1) for August 2010, referenced against the climatological mean. The black contours outline the patches where significant anomaly (>2 standard deviations) was found. The 20% surface oil presence line is annotated in white in Figure 1a and brown in Figure 1b. The three R/V WB‐II stations are marked with “+”. (c) The inset shows the same anomaly as in Figure 1b but referenced against the climatological maximum for August of 2002–2009.

resuspension is restricted to nearshore waters [Salisbury et al., 2004]. Under such circumstances, several studies have shown that MODIS fluorescence line height (FLH) was relatively stable against CDOM changes and could be used as a better proxy for phytoplankton biomass [Hu et al., 2005; McKee et al., 2007]. For example, McKee showed that for a 20‐fold increase in CDOM, FLH decreased by at most 50%. Thus, given the 2–4‐fold changes in CDOM for the NEGOM [Hu et al., 2003a; Nababan, 2005] and relatively stable chlorophyll-a fluorescence efficiency (MODIS/ Aqua measurements are within 1–2 hours of solar noon), MODIS FLH was assumed to be nearly immune to such changes and consistent across different years for the purpose of assessing inter‐annual changes in phytoplankton biomass (water greenness). [7] MODIS monthly data at 4‐km resolution were obtained for the period of July 2002 to January 2011 from the U.S. NASA Goddard Space Flight Center. These included: FLH normalized to the same incident irradiance (in mW cm−2 mm−1 sr−1), night‐time sea surface temperature (NSST, °C), photosynthetically available radiation (PAR, mol photons m−2 day−1), and remote‐sensing reflectance at 667‐nm (Rrs667, sr−1). The products were derived using the most recent updates in calibration and algorithms in the software package SeaDAS (version 6.1), and were used to generate monthly climatologies, monthly means, standard deviations, and anomalies. If the anomaly deviated from the climatology by >2 standard deviations, it was regarded as statistically significant. For the identified anomalies, 3‐day products were obtained and examined for episodic events. For each month, a 2002–2009 climatological monthly maximum was also derived in order to evaluate whether FLH in 2010 exceeded the historical maximum. [8] The surface oil location was determined from the daily MODIS data (250‐m resolution) and MERIS data (300‐m resolution) between 22 April and 31 July 2010 [Hu et al., 2003b, 2009], complemented by some radar observations. Because it was impossible to estimate the oil film thickness, we simply used oil presence/absence to estimate the frequency of oil appearance during this period, normalized by the number of valid observations. In addition, numerical

circulation models were used to estimate the potential trajectories of surface and subsurface oil [Liu et al., 2011].

3. Did the NEGOM Become Greener? [9] Figure 1a shows the percentage of days of surface oil observed from 22 April 2010 (the first day after the oil platform sank) to 31 July 2010 (when surface oil expression disappeared in satellite imagery) from 58 cloud‐free images. The surface area affected by oil is about 103,000 km2, most of which (∼96,000 km2) is in the NEGOM (bounded by 27.5°N to 31°N and 91°W to 82.6°W). This corresponds to nearly half (45%) of the entire NEGOM water area (∼212,000 km2). About 21,800 km2 of ocean surface area was exposed to oil for >20% of the time (white outline in Figure 1a), while about 3,600 km2 showed oil for 50% of the time. These results clearly indicate large‐scale oil contamination in both time and space. [10] Figure 1b shows the MODIS FLH anomaly for August 2010, the first month after the surface oil disappeared. A large, contiguous patch (∼11,100 km2) of a significant positive anomaly was observed in the eastern side of the region bounded by the 20% surface oil line. Most of the patch had FLH anomalies >0.01 mW cm−2 mm−1 sr−1, corresponding to roughly 1 mg chlorophyll‐a m−3 [Hu et al., 2005]. Figure 1c further shows that in this patch FLH values are higher than in any August between 2002 and 2009, suggesting that 11,100 km2 of the NEGOM water was significantly greener during August 2010. Even when all data between July 2002 and March 2010 were combined together, about 27% (3,000 km2) of this patch still showed higher FLH value than the cumulated maximum. Several statistically significant smaller patches also appeared in the vicinity and to the southwest of the Mississippi River mouth, but they did not show similar spatially coherent patterns. The 3‐day composite FLH and FLH anomaly images between late July and early September 2010 were examined to see whether this large anomaly patch was persistent and coherent during August. Although frequent cloud cover and sun glint created data gaps and image patchiness, the anomaly indeed started from early August,

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Figure 2. (a) MODIS FLH anomaly (mW cm−2 mm−1 sr−1) for December 2010. The black lines denote isobaths of 10, 20, 30, 50, 100, and 200 m, respectively. Most of the positive anomaly between 30 and 100‐m isobaths are above 1 standard deviation but 2 standard deviations). (c) The inset shows MODIS Rrs667 anomaly (sr−1), where most yellow and red colors (>1 standard deviation) along the west Florida’s coast are within 20‐m isobaths. lasted for most of the month, and disappeared in early September. [11] There were no similar significant FLH anomalies (either positive or negative) between April and July or after August. During December 2010, a coherent positive FLH anomaly was observed extending from Mobile Bay to the Florida Keys, mainly between the 30 and 100‐m isobaths on the west Florida shelf (WFS, Figure 2a). This positive anomaly was above 1 standard deviation, but not statistically significant. Concurrent MODIS SST data showed a significant negative anomaly (Figure 2b) corresponding to the same regions of the positive FLH anomaly. More recent data showed that the FLH anomaly lasted through January 2011 and became statistically significant. [12] Two questions thus arise from the above observations: 1) Is the significant FLH anomaly in August 2010 due to the DHW oil spill? 2) Is it possible that some of the subsurface oil and/or dispersants contributed to the positive FLH anomaly in December 2010 and January 2011, via ocean circulation and a combination of upwelling and mixing?

4. Causes of the FLH Anomalies 4.1. The August 2010 Case [13] Data collected from nine oceanographic cruises in the NEGOM between November 1997 and August 2000 suggest that surface chlorophyll‐a distributions are strongly influenced by river discharge [Hu et al., 2003a; Qian et al., 2003; Nababan, 2005], where in situ Chl in the western part of NEGOM typically ranged between 0.2 and 2 mg m−3. Discharge data from the two major rivers (the Mississippi and Apalachicola rivers) collected by the USGS in the past decades, however, did not reveal a significant anomaly during summer 2010, and river discharge anomaly did not correlate to FLH anomaly (R2 < 0.06 for the 26 summer months and 20 mm) was dominated by oceanic diatom and dinoflagellates species (5,333– 405,833 cells L−1), with suppressed activities in microbial and phytoplankton communities (Paul et al., submitted manuscript, 2011). No toxic species (Karenia brevis, Pseudo‐nitzschia sp) were present. Preliminary analyses of zooplankton abundances indicate a high spatial variability,

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Figure 3. Modeled particle trajectories on 31 May 2010. The particles were released at (a) 400 m and (b) 1200 m every 3 hours starting from 21 April (blue color, i.e., initial particle release or 0 days) through 31 May 2010 (red color, 41 days). The WFS ROMS model was forced by realistic winds and nested into the global HYCOM model to obtain boundary conditions. (c) Climatological (1994–2010) monthly mean winds (blue) and their standard errors (black) versus 2010 monthly mean winds (red) measured at NDBC buoy 42036 on the northern WFS. Mean wind in December 2010 is in the along‐shelf direction (i.e., parallel to Florida’s west coast) and much stronger than the December climatology, therefore more effective in driving the shelf‐wide upwelling. with surface (20 m) zooplankton densities ranging from about 10,000 to >70,000 individuals m−3. Thus, complex ecosystem interactions may account for the observed positive (significant) and negative (insignificant) FLH anomalies in August 2010. 4.2. The December 2010–January 2011 Case [16] The FLH anomaly showed a discontinuity between offshore (100–30 m) and inshore (20–0 m) regions. Inshore, the Rrs667 anomaly (Figure 2c) indicates that a significant sediment resuspension occurred due to strong winds (Figure 3c). Thus, the high FLH anomaly in nearshore waters suggests high turbidity as opposed to high biomass [Gilerson et al., 2007]. In contrast, the offshore FLH anomaly from Mobile Bay to the Florida Keys appears to be related to enhanced phytoplankton biomass. [17] A spring phytoplankton bloom typically occurs south of Apalachicola Bay and is referred to as the “green river” [Gilbes et al., 1996]. The origin of this feature is related to seasonally varying circulation [Weisberg et al., 1996; He and Weisberg, 2002], driven by both varying winds and surface warming. Anomalous forcing, by either anomalous winds or the Loop Current and eddy interactions with the shelf slope, may further accentuate circulation and upwelling processes, increasing advection and mixing of deep GOM nutrients across the shelf break and onto the continental shelf [Weisberg and He, 2003; Walsh et al., 2003]. An early onset of strong weather fronts led to anomalously strong upwelling favorable winds in December 2010 (Figure 3c) [Weisberg, 2011]. Thus, the spatial extent of the bloom (from Mobile Bay to Florida Keys) and the concur-

rent SST anomaly suggest that the bloom was the result of upwelling and mixing of nutrients across the shelf break. [18] Although circulation models indicate that subsurface oil may have spread southward along the slope of the WFS [Liu et al., 2011], it is uncertain whether or not concurrent upwelling and mixing of subsurface hydrocarbons and dispersants contributed to this anomaly owing to a lack of in situ data. Systematic sampling is needed from the head of Desoto Canyon southeast along the shelf break and onto the shelf, because this is where deep ocean nutrients are advected onto the shelf. Clues may eventually be found in the sediment samples.

5. Conclusion [19] The use of synoptic, consistent, and multi‐year MODIS FLH data product is believed to provide a reasonable proxy for ocean greenness (phytoplankton biomass) in the optically complex NEGOM region, which led to the detection of a significant positive anomaly during August 2010. Yet the argument that the anomaly is due to the spill is based on indirect evidence through satellite observations, modeling, and ruling out other possibilities. Unfortunately, field observations before and after the spill were too scarce to provide direct evidence on what biogeochemical processes led to the observed biomass changes. Such a lack of adequate data to assess the impacts of the DWH oil spill highlights the need for sustained ocean observations, especially when the chronic biological effects of the DWH oil spill are to be assessed. The optical complexity in the NEGOM also calls for improved remote sensing algorithms

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to better estimate chlorophyll‐a concentrations and other biogeochemical properties. [20] Acknowledgments. Funding for satellite work was provided by the US NASA Ocean Biology and Biogeochemistry (OBB) program and Gulf of Mexico program. We thank the NASA Ocean Biology Processing Group for providing satellite data. We also thank Jennifer Wolny and Sue Murasko (Florida Fish and Wildlife Research Institute) for collecting and analyzing phytoplankton samples. WFS observing and modeling activities benefited from many funding sources, including the State of Florida, NOAA, ONR, NSF, and NASA. We also acknowledge support from the University of South Florida Research Foundation grant 94302 and FIO‐ Deep Water Horizon Oil Spill grant 4710‐1101‐01 to KLD. The author thanks two anonymous reviewers for their assistance in evaluating this paper. [21] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper.

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