4.1 The Atmospheric Conditions of the Marine and Coastal Environment
Stewart R. H., (2008 pp39) noted that because the atmosphere drives the ocean, while the ocean drives the atmosphere, it is imperative to consider the ocean and atmosphere as a ‘coupled dynamic system’.
Observation of Wind Patterns: In general, the atmospheric characteristics underlining the coastal and marine environment of Ghana have minimal variations, thus according to earlier EIA baseline studies conducted by ERM/TGL (2009) and ESL Consulting/ENI S.p.A (2015). The ERM/TGL (2009), asserts for example that, primarily, observed winds generated are of the southwest quadrant –and thus having a maximum non-squall attribute of 10m/s wind speed. Fig. 3 also shows results compiled of daily wind from the study region during the study. Winds on the sea surface usually represent velocity structure near the base of the atmospheric boundary layer (ABL) and are directly linked to circulations above the ABL with patterns reflecting the entire structure of the atmospheric general. Therefore, at low latitudes, trade winds dominate while the easterlies winds dominate the Equator in both the Pacific and Atlantic Oceans (Rui Xin Huang, 2017; 2015).
Again, the assertion from ERM/TGL (2009) and ESL Consulting/ENI S.p.A (2015) is in agreement with the findings of the study depicted in Fig. 3 and 4, which showed an intense wind activity directed within the southwest quadrant of the wind rose, with maldistribution within the northwest. Again, 41.8 per cent wind speed recorded between the 10th of January and the 28th of March 2020 study period, fell within the frequency range of 7-11knots. ERM/TGL (2009) notes that these wind characteristics accordingly, beset the fairly consistent year-round wind patterns of the region. Squall[1] events observed, however, have been attributed to thunderstorm generating extreme wind conditions. Thus, approximately 15 – 30 squall events are expected yearly though with a short duration. Implicitly, over the ocean, they end up with weaker surface current and low wave heights (ERM/TGL, 2009). For coastal areas, with time, wind stress changes over most parts of the world’s oceans to a noticeable seasonal cycle (Rui Xin Huang, 2017; 2015). It was observed that the coastline areas show high levels of atmospheric salt precipitations largely due to high temperature and convectional wind activities near the surface.
Observed Atmospheric (Barometric) Pressure: the period of January to March 2020 for which the study carried out before the covid-19 pandemic (Verdict Media Limited, 2020), showed relatively, a minimum and maximum sea-level pressure (MSLP) range of 1000mbar and 1010mbar (shown in Figure 5) — thus, below the standard average sea-level pressure of 1013.325mbar (101.325kPa; 29.92 inches of mercury; 760.00mmHg per the International Standard Atmosphere (ISA). The GCNS (2009); Stewart (2008) noted that pressure distribution around the earth surface results from the heating sun (insolation) which varies with latitude.
From the graph, the least sea-level pressure of 1000mbar recorded was by the noon hour (GMT) midday. Pressures recorded remained reasonably low –influencing atmospheric activities (such as cloud and winds resulting in the occasional rainfalls observed over the study period). The 6:00 am, 12:00 am and 12:00 pm periods recorded as high as 1010mbar of barometric pressure— with a Mean (average) at 1006.578947mbar. The region under study lies near the equatorial latitude, and hence, tends to benefit from the massive influx of the sun’s thermal insulation –resulting in surface heating. The area, therefore, lies within the ‘Equatorial Low’ (GCNS GCM, 2009). These recorded pressure are said to influence the amount of dissolved oxygen along the ocean column which helps maintain the function living of organisms that require oxygen for metabolism.
Observed Conditions of Visibility: in navigational terms, visibility means the extent to which an individual either on the navigational bridge or deck watch-keeping can maintain a visual lookout for an object over distances on ocean up to horizon form a marine vessel or platform. Brown and Brown, (2016) explains this as the “term used in describing the transparency of the atmosphere, and defined by the maximum distance at which a suitable object is seen.” Conditions such as fog, sea smoke and massive rainstorm are a few atmospheric processes that contribute to reducing visibility during the day. Implicitly, the knowledge of the level of visibility on the ocean at any particular time is paramount in ensuring the safety of life, property and environment during navigation. Figure 6 is a graph indicating a visibility pattern over the study region for the study period.
Though visibility recorded was relatively good (mostly) with a median of 10nm, the minimum visibility recorded over the period was under 2nautical miles (nm) in January and February. The reduced visibility recorded can be attributed to both radiation and advection fog activity. The radiation fog influenced the visibility of the vessel at the Takoradi port location from the 10 through to the 12th (during the P.O. vessel mobilisation). Unlike the radiation fog formation occurring at the coast of Ghana, the remoteness of the condition’s location over 60kilometers offshore, suggested an advection fog formation (Brown and Brown, 2016) –thus, consistent with the low visibility through the first and second weeks of February heading into March (marking the summer cold sea temperatures). This condition poor visibility, therefore, may put animals; surfacing on the water in the region, at risk of strikes from speeding marine vessels. Maximum visibility was as high as 12nm, though did vary over 10nm for the study period. The period under study also marked the upwelling season characterising the feeding behaviour of marine mammals in the area –suggesting the imminence of animal surfacing behaviour.
Atmospheric temperature: which is the measure of the degree of hotness or coldness of an object or place, was primarily crucial in this study in understanding the level of thermal energy transfer occurring above the ocean surface within the study location. Stewart R. H. (2008) in fact, asserted that only a fifth of the solar energy released by the sun is absorbed directly into the atmosphere as either evaporation or infrared wave energy. Therefore, the temperature seen in Figure 7 is a recorded function of these two primary atmospheric energy delineating the atmospheric dynamics of the study region within the subtropical area.
From the graph, temperatures from the 10th of January and 28th of March at 0600 am (hr) range between 270C and 300C with a Median of 29 0C. Similarly, the 1200pm (hr), 1800pm (hr), and 0000am (hr) recorded minimum and maximum temperatures of 260C – 320C (a median of 300C); 260C – 310C (a median of 300C); and 280C – 300C (a median of 29.50C), respectively. These results are fairly consistent with sub-tropic temperatures readings –suggesting some levels of active humidity. These temperatures conditionally influence marine animal’s surfacing and deep-dives from time to time.
4.2 The Ocean Surface Conditions
Observed Wave Activity: the waves were driven by surface wind interactions – generating significant wave heights up to 1.4meters. Waves with a median of 0.9meters. These are represented in Figure 8. Wave patterns offshore and near coast showed the similar characteristic of small amplitudes, however, multi-directional offshore compared to near coast waves that ended up on the beachlines. Given that small-scale surface waves first are generated from wind stress; subsequently, via ‘wave-wave interaction,’ energy released in the phase space –resulting in surface waves with a long wavelength and large amplitude were not commonly seen in the region. This, Rui Xin Huang (2017; 2015) compared to larger-scale currents, that indicated surface waves— thus, concluding small-scale wave is the problem.
The maximum recorded wave was over 2meters in height for January through to March 2020. These waves characterised were associated with the southwestern quadrant –the same pattern as with winds. However, wave periods over time are subject to dynamic instability in which stable ocean waves become unstable due to velocity shear.
Observed Ocean Current: activities of ocean current observed was of both surface and water current conditions, primarily referred to as Guinea Surface Current and Guinea Underwater Current. The surface current measured was from the 0meter ocean depth at the surface to a depth of 40 meters. Figure 9 shows both the amplitude and direction of the water surface current. The direction of the surface current showed a powerful feature within the southwest and northwest quadrants. According to GCNS (2009), surface currents are mainly wind-driven (see also Fig. 3 and 4) –a conduit of surface friction at a right angle due to the Coriolis Effect causing the current heading in the westerly direction. The patterns observed compared favourably with ERM/TGL (2009) and ESL Consulting/ENI (2015) findings, as far as the depiction of global surface ocean current from GCNS (2009), is of concern.
Again, Stewart (2008 pp51) notes that tropical ocean currents, particularly of the Atlantic, play a vital role in the unsteady transportation of thermal energy (solar energy stored in the ocean) from summer to winter during the amelioration of the earth’s climate. This reason is also critical to understanding climate change.
The dynamics between the current and waves observed in the earlier graphs was paramount to understanding ocean stability. Hitherto, Stewart (2008 pp129) stressed that because oceans appear strongly stratified with weaker currents, turbulent mixing remains intermittent and are rare. This situation generates a period of the stable and unstable ocean surface from frequent wave formations and breaks.
4.3 The Ocean Structure and Marine Ecosystem
The ocean is a natural supporter of various life forms –with the capabilities to thrive in its unique environment. However, the life support of most living mammal or organisms relies on fundamental environmental conditions that include Oxygen, lamination (Sunlight), water, favourable shelter and food sources. The ocean, unlike rivers and streams that are freshwater sources for animals within, living organism in the ocean, are forced to contend with a high concentration of salts. The study primarily examined the relationship of salinity concentration (psu), dissolved oxygen (D.O.) concentration (mg/L at 100% saturation level), Density (D) and temperature variations (synonymous to radiated heated energy from sunlight) over water depth as characteristic supportive of marine lifeforms in terms of feeding, habitat, and procreation.
4.3.1 Water Column and Hydrographical Data
While previous environmental baseline surveys (EBS) conducted in EIA over the region on the various oil fields and surrounding areas have determined to some level the baseline conditions of the environment. Thus on biological, chemical and physical importance before field developments, it also identifies ecosystem parameters sensitive to impending changes; thus, all as part of meeting environmental impact assessment requirement, the EBS sampling locations were selected by the oil field license operators in their EIA EBS. The scientists factored in; daily water column profiling, and seafloor sediment sampling (CSA International, Inc. /TGL, 2011).
The summary hydrographic results of the 2011 EBS on the DWT block by CSA International, Inc. /TGL, (2011) which is also within the field demarcation of this study, have been presented in detail alongside the findings of sample location selected in this study, to enable comparative analysis. The sample data accompany graphical analysis. The other samples were from the WCTP license block, Jubilee oil field area, which also forms part of the study location. The EIA EBS carried out was conducted by TDI-Brooks International aboard R/V JW Powell between 09th through to the 13th of September 2008. The study at this point does consider graphical representation in Figure 10 which represents hydrographical data collected over the TEN and Jubilee fields at varying points in time.
The two graphs of TEN-EBS Station-4 and Jub-EBS0003 attempts to illustrate the relationship between Salinity, temperature and oxygen of the sampled location. (See supplementary material for tabular recorded data).
The graphs (in Figure 10), represent parameter data obtained at varying timelines, thus, EBS Station -4 on the 26th march 2011 and sample station EBS003 on the 12th of June 2008 – marking a three-year interval of data timelines. Both locations fell within a 1400 and 1500meter water depth zone. Sea temperatures observed varied between 24.80 C to 4.20 C (at Jub EBS 003) and 21.050 C to 4.150 C (EBS Station 4) respectively –and in both cases decreasing along with the water depth. Juxtaposing the above graphs (Figure 10) of March 2011 and June 2008 site data recordings to Figure 11, which are sample analysis graphically produced based on 2020 data gathered on-site study, the assertions in the proceeding paragraphs can be made. Thus, the real sense or level of variations can be determined.
The Jubilee P1 Manifold location (Figure 11 on the right: data taking within February 2020) had a maximum water depth range of 1235meters (with a median water-depth of 621.5, and standard error (S.E.) of 10.12011199), from which various hydrographical data was as obtained. At a minimum of 8meter water depth, sea surface temperature (SST) was 29.99-degree Celsius (S.E. = 0.147263907, and a median temp. of 6.31degree Celsius, Sample Variance =26.63121632). A minimum temperature of 4.54-degree Celsius recorded was also at the maximum water depths of 1235meters. Salinity readings also did vary between a maximum and minimum of 35.27psu and 33.74psu (with a Median = 34.42, S.E. = 0.007588667), whereas max., and min., densities (D) are 33.2522 and 20.8852 kg/m3 (with a Standard Error = 0.071268403, and a Median = 29.75945). Dissolved Oxygen (D.O.) estimated over the area showed a median value of 8.934279203mg/L (S.E. = 0.01663382) with a Minimum of 6.267872443mg/L and a Maximum of 9.169776741mg/L at a 100% saturation value under 1009.6mbar of atmospheric pressure.
Similar patterns observed are of the Jubilee FPSO location (see Figure 12) area with a 1002meter water-depth, a minimum temperature of 4.8-degrees Celsius and a maximum of 30.2-degree Celsius
A 34.12psu and 35.39psu as well as 21.0411 and 31.9387 kg/m3 are the minimum and maximum Salinity, and density values recorded respectively across the water depth— with estimated D.O. ranging between 6.240481615mg/L and 9.173744832mg/L of 100% saturation under 1005mbar of atmospheric pressure.
Additionally, slight variations observed, are in Jubilee-field locations KP0.000 (in Figure 12: data taking in March 2020), and KP48.808 (in Figure 13: data taking in September 2020) over the study period. Again, profiling for chemical and physical parameters in the EIA survey by CSA International, Inc. /TGL, (2011), revealed in total that nitrogen and phosphorous were present in all samples though with minor differences among samples. Differences were also evident in chemical composition for samples collected at deeper depth compared to shallow areas.
Thus, the sample from deeper depth (i.e., at 100m or near the bottom) regardless of the sample stations, recorded <0.24 to 0.40 and 0.0335 to 0.0545 mg/L total nitrogen (N) and phosphorous (P) concentrations respectively compared to <0.15 to 0.22 and 0.0141 to 0.0161mg/L of N and P. Hence, there were no viable differences in chlorophyll and pheophytin concentrations compared to nutrients near the surface and at depth— a subject of productivity and organic mineralisation (CSA International, Inc. /TGL, 2011).
The lack of difference according to CSA International, Inc. /TGL, (2011) suggest the near-surface water column as beset with the same water mass.
Therefore, CSA International, Inc. /TGL, (2011) also stated that, given that the hydrographical parameters were consistent with measured water quality parameters from the various EBS, it was indicative of the general open ocean conditions underpinned by low levels of chlorophyll, suspended solids and nutrients in general. Comparatively, the rest of the selected stations for reanalyses are present in Figure 14, and Figure 15, which covers surveys conducted in 2008. The data also did concur with the findings of this study in terms of Salinity, DO, Temperature.
The results across 2008, 2011 and 2020 surveys showed very little variation in terms of average sea temperatures, salinity, and density. Thus, considering the thermocline divide by temperature, the halocline divide by salinity levels, and the pycnocline divide by density, according to Fondriest Environmental, Inc., (2013) each of these clines can affect the amount of dissolved oxygen the ocean strata hold. This could in part be attributed to the regions fairly consistent chemical composition across the sea area regardless of the water-depth and composition of the seafloor. Dissolved Oxygen (DO) in the thermoclines regions were higher at 100 per cent saturation. This was not the case for stations with relatively shallow water under 100 meters. The high concentration of DO along the water column of EBS stations with deep-water depth (1000meter above) is in part due to strong offshore wind circulations on the ocean surface. The result also showed that there were no twilight zones (which refers to intercessions) of salinity and temperature samples collected at deep-water enclaves (see Fig. 14, 15) compared to relatively shallow waters (see Fig. 11, 12, and 13), which showed DO levels slightly below 5mg/L at relative constant with little variation. Thus though locations with such shallow depth tend to be tidal with fluctuating dissolved oxygen levels where they are near estuaries (Fondriest Environmental, Inc., 2013), the results amplify a region with decline DO— suggesting strong usage by colonies of coastal benthic organisms and pelagic fishes.
The Sound Velocity Gradients: for the sample locations in the 2020 Jubilee study, the researcher also found that the sound velocities showed similar patterns of graduation summarised in Table 1 and Table 2. (See graphs in supplementary material)
Table 1: Sound Velocities computed for three stations between February and March 2020.
Descriptive Statistics Terms
|
JUBILEE STATION IDENTIFICATION
|
P1 (10 Feb 2020)
|
B5 (15 Feb 2020)
|
B6 (15 Mar 2020)
|
Depth(m)
|
Velocity(m/s)
|
Depth(m)
|
Velocity(m/s)
|
Depth(m)
|
Velocity(m/s)
|
Mean
|
505
|
1507.677709
|
505
|
1495.922824
|
493
|
1495.33529
|
Standard Error
|
9.110434
|
0.343496926
|
9.110434
|
0.433606446
|
9
|
0.41149073
|
Median
|
505
|
1504.71
|
505
|
1490.7
|
493
|
1489.38
|
Mode
|
#N/A
|
1526.42
|
#N/A
|
1485.37
|
#N/A
|
1485.13
|
Standard Deviation
|
287.3761
|
10.83513671
|
287.3761
|
13.67751722
|
280.4479
|
12.8224105
|
Sample Variance
|
82585
|
117.4001875
|
82585
|
187.0744774
|
78651
|
164.41421
|
Kurtosis
|
-1.2
|
-0.368146391
|
-1.2
|
1.464434072
|
-1.2
|
1.52001305
|
Skewness
|
8.95E-19
|
0.805357118
|
8.95E-19
|
1.377590478
|
-6.2E-17
|
1.42319639
|
Range
|
994
|
39.77
|
994
|
61.23
|
970
|
58.36
|
Minimum
|
8
|
1494.97
|
8
|
1484.01
|
8
|
1484.84
|
Maximum
|
1002
|
1534.74
|
1002
|
1545.24
|
978
|
1543.2
|
Sum
|
502475
|
1500139.32
|
502475
|
1488443.21
|
478703
|
1451970.57
|
Count
|
995
|
995
|
995
|
995
|
971
|
971
|
Table 2: Sound Velocities computed for two stations in September 2020.
Descriptive Statistics Terms
|
JUBILEE STATION IDENTIFICATION
|
Station B2 (05 Sep 2020)
|
Station BX (22 Sep 2020)
|
Depth(m)
|
Velocity(m/s)
|
Depth(m)
|
Velocity(m/s)
|
Mean
|
377
|
1498.476762
|
360.5
|
1509.539088
|
Standard Error
|
7.46101
|
0.409135915
|
7.653975
|
0.264031256
|
Median
|
377
|
1495.25
|
360.5
|
1508.67
|
Mode
|
#N/A
|
1486.94
|
#N/A
|
1521.39
|
Standard Deviation
|
192.6906
|
10.56648452
|
202.7942
|
6.995582733
|
Sample Variance
|
37129.67
|
111.6505952
|
41125.5
|
48.93817777
|
Kurtosis
|
-1.2
|
-1.274934095
|
-1.2
|
-1.33726051
|
Skewness
|
-6E-17
|
0.490753374
|
-4.1E-17
|
0.168369103
|
Range
|
666
|
31.85
|
701
|
22.3
|
Minimum
|
44
|
1486.68
|
10
|
1499.09
|
Maximum
|
710
|
1518.53
|
711
|
1521.39
|
Sum
|
251459
|
999484
|
253071
|
1059696.44
|
Count
|
667
|
667
|
702
|
702
|
The study found the median sound velocity range of 1489.38 m/s and 1508.67m/s over varying water depths ranging from 8meters to 1002meter depth of the various sample locations. The findings are in agreement with previous data which categorises sound velocity at 1500m/s –allowing animals using sound to detect target prey (during echolocation), achieved their goal with wavelength 4 or 5 times to length of target prey (Project Oceanography, 2000). It is also useful in animal navigations, feeding and breeding.
4.3.2 Habitat Found in the Region and Vulnerability Concerns
The nature of the habitat observed was based on animal sightings that took place throughout the study. This period refers to the January to March 2020, and the September 2020 field observations. Other observations of species made include the periods of March 2017 to March 2018 on the offshore Sankofa Gye Nyame oil and gas field; and September to November 2015 on the TEN oil and gas fields. Among some of the animals observed, interest includes pelagic fishes, sharks, Short-finned Pilot whales, Bottlenose Dolphins, Humpback Whales, Marine Turtle, and Mantra. Bottom dwellers also observed included worms and pink coloured jellyfishes at locations as deep as 700meters on the Sankofa Gye Nyame field. Again, observation of offshore subsea construction operations over the period did appear to have risk implications in terms of sound (underwater noises), movement, night-time lightings, flaring and pollution. Several sightings of Mantra like that seen in Figure 16 occurred at over 100meter water depth which corresponded with the high levels of dissolved oxygen concentrations.
Table 3 here list some of the essential animals observed. Seabirds were a continuous presence through the period of September on feed hunting routs.
Table 3: Some Frequently Observed Animals
Month of Observation
|
Animals Sighted
|
January
|
Bottlenose Dolphins
|
January, February, March, September
|
Short-finned Pilot Whales
|
February
|
unidentified cetacean
|
September
|
Seagull
|
February, September
|
Mantra
|
March September
|
Marine Turtle
|
June, July, August
|
unidentified dolphins
|
February
|
Sharks
|
All months during the survey
|
Variety of Fishes
|
Source: field survey
Given that the area lies directly in the heart of the West African shipping lane as depicted in Figure 17 (http://www.nceas.ucsb.edu/GlobalMarine/impacts, 2009; ERM, 2009), there are real vulnerabilities if measures are not adequately designed to cater for the growing changes. Other vulnerabilities identified were the concern of marine plastic particularly along the coast and oil spill (Lamptey and Sackey, 2017).
[1] Squall is a sudden violent gust of wind or localized storm, especially one bringing rain, snow, or sleet.