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
|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.|
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.
|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.|
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).
|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.|
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.
Automated Tools for Kelp Detection and Mapping
Kelp, a widespread marine macroalgae is also the focus of global conservation efforts. Kelp can grow to be very tall and provide habitat for a variety of nearshore marine life. We have seen great success by researchers like Rick Ware from Coastal Resources Management Inc using Lowrance HDS and BioBase EcoSound to map kelp forests of the US Pacific Coast (Figure 8). As seen in the Broadband 200 kHz sonar log on the right, EcoSound detected Kelp up to 9.8 m (32 ft) long! Ground truth data collected by Mr. Ware confirmed this acoustic signal was indeed kelp. This trip is also publicly available in our demo account.
|Figure 6. Kelp beds off the Southern California Coast detected by Lowrance HDS and BioBase EcoSound and confirmed by researchers.|
Let us know how you are using BioBase to address questions in tidal influenced habitats.