Although BioBase EcoSound was originally developed for aquatic vegetation mapping in inland lakes, users along both US Coasts have helped us diversify its toolbox to now be a powerful coastal habitat mapping tool as well!
One of the biggest challenges of mapping coastal habitats is their tidal influence with depths changing harmonically based on the moon phase and other factors. Fortunately, however, widespread tide stations and large public databases of tide predictions allow for accurate and precise offsets to georeferenced and time-stamped sonar logs from Lowrance HDS or Elite units uploaded to BioBase EcoSound. BioBase EcoSound immediately queries the nearest tide station to your upload (up to 75 km) and adjusts your depth and seagrass or kelp biovolume to the Mean Lower Low Water (MLLW) datum every 5 minutes. Tidal statistics (Avg., start, stop, high, low,) are archived in your account for each trip.
Simulate Tidal Conditions with the Data Offset Tool
Figure 1. demonstrates what seagrass density conditions looked like in a harbor in Newport Bay CA USA on 12/5/2012 at MLLW (Data collected by Rick Ware; Coastal Resources Management Inc., Corona Del Mar, CA). Areas of red indicate where seagrass or eelgrass (Zostera marina) grows to the surface at MLLW and may interfere with the passage of vessels in and out of the Marina. Thus, these habitats may be high risk of damage. Full access to the trip displayed in Figure 1 among other trips, is available for free to anyone in our public demo account.
We can also simulate what this same area looks like under high water conditions (Mean Higher High Water – MHHW) using the data offset tool in BioBase (Figure 2). MHHW data were retrieved from NOAA and the average tide (1.5 ft) for the hour that data were collected in this area in Newport Bay was added to the MLLW average offset (3.84 ft; Figure 3).
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| Figure 1. Mapping tracks (red lines) overlain on a seagrass density heat map with depth contours. Areas of red are where seagrass grows close to the surface at low tide (MLLW). Areas of green are low-lying seagrass areas. The cross section of the sonar log displaying the high resolution imagery of StructureScan is displayed on the right and synchronized with the map upon each upload (Trip Replay). Data were collected with Lowrance HDS and uploaded to BioBase |
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Figure 2. Data Offset tool in BioBase EcoSound can be used to offset depths to a benchmark elevation (e.g., full pool for reservoirs), or custom tides as is the case here. MHHW data were retrieved from the Newport Bay Entrance Tide Station and added to the MLLW offset to simulate high tide conditions
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| Figure 3. Trip-specific tide data retrieved from the NOAA Tidal Prediction web page. 60-min values during the precise hour of data collection were added to the MLLW offset in the Data Offset tool in BioBase EcoSound. |
Compare effects of tides in the automated reports
Comparing results from the automated reports (Figures 4 and 5) one can see the effect that tides can have on biovolume and potentially seagrass habitat conservation. Notice that percent vegetation cover (PAC) does not change much with tides, but biovolume (BVp), which brings in the 3rd dimension of plant height, does.
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| Figure 4. Excerpt of automated report for MLLW conditions in Newport Bay. Statistics on survey area, water volume, vegetation summary statistics, waypoint summaries, and important metadata are available with each upload. |
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| Figure 5. Excerpt from BioBase EcoSound trip report with a MHHW offset added to the output. PAC (grid). Compared with MLLW data in Figure 4, PAC only increased from 64.5% to 71.3% but average biovolume in vegetated areas decreased from 38.8% at MLLW to 16.7% at MHHW. |
Interpreting outputs in extreme tides
In some areas of the Pacific Northwest it is not uncommon to see 10 ft tides. Mapping seagrass at high tide in areas high and dry at low tide, represents a great opportunity to passively map the cover and density of seagrass beds. In areas exposed at low tide, BioBase Ecosound will reclassify any seagrass detected to 100% biovolume, displaying a red area of seagrass coverage (Figure 6).
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| Figure 6A. Map displaying density of seagrass beds at MLLW in Puget Sound, WA USA with a 9.3 foot average tide. 200 khz Sonar chart on right shows the actual depth during the time of assessment and seagrass growth on bottom. |
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| Figure 6B. Zoom of Figure 6A and display of tide-corrected point data along boat track (red line). Highlighted cell corresponds to the orange dot on the map on the left and the area just to the right of the faint white line on the sonar graph on the right. If the seagrass plant height is greater than the tide-corrected depth, the bv registers a n/a and it is considered 100% (red) in the biovolume map. If plant height is less than the corrected-depth, biovolume is recalculated as the proportion of plant height to corrected water depth. Zero plant height and biovolume areas remain |
Use the Polygon Tool to delineate and monitor sensitive seagrass areas.
Figure 7 demonstrates how a polygon could be drawn and used to calculate seagrass cover statistics in a specific area and time. You can use the same polygon on subsequent trips to monitor change over time and evaluate protection or restoration.
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| Figure 7. The Polygon Tool was used to draw a boundary around a bed of seagrass in a shallow area of Puget Sound, WA USA where boat traffic is high. The same polygon can be used to monitor change across multiple trips in the same area (users should take reasonable measures to travel the same path repeatably to generate the most comparable results) |
Automated Tools for Kelp Detection and Mapping
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| Figure 6. Kelp beds off the Southern California Coast detected by Lowrance HDS and BioBase EcoSound and confirmed by researchers. |
