There are many types of monitoring efforts using different methodologies. Many are based on the same techniques with slightly different protocols. For example, the National Park Service uses a Tiered approach, where Tier 1 is a broad spatial scale giving you overall seagrass distribution (see the Mapping subcategory for discussion about those methods), Tier 2 involves in water surveys giving you seagrass composition and cover, and Tier 3 which provides an avenue for hypothesis testing that can get at why changes in Tier 1 and Tier 2 sampling are observed. The question then becomes is this the best framework for global seagrass monitoring, given costs and amount of labor involved.
We use this protocol for the NPS monitoring of Fire Island National Seashore. The great drawback is without the Tier 1 monitoring, you loose the ability to see overall changes in distribution. If the lack of funding for flights or poor water clarity prevent the large scale assessment, the Tier 2 is insufficient to document change unless it is very significant. I was greatly flustered by the inability of the Tier 2 to assess changes from our barrier island breach from Superstorm Sandy where I would drive the boat through patchy grass to record “no seagrass” at my Tier 2 site. This is why I’ve become a strong advocate for drone imagery to “fill” the gap between Tier 1 and Tier 2 to better assess change at sentinel sites across the boundary of the park. The Tier 1 imagery was flown in 2002 and the next in 2018 making the ability to address mechanism very difficult.
I’m mostly familiar with the tiered approach that Dottie described, which is what I use for long-term seagrass monitoring. I’m interested to hear about other methods that may not require aerial imagery for identifying broad distribution changes, and are therefore less costly and perhaps more widely useable.