Apologies for top-posting, but the interface in the browser does not enable me 
to make any sense of multiple comments. Anyway here goes:
Zostera (eel-grass) grows below the tide line, so really is not an emergent 
plant. Other things in your list of aquatic bed vegetation are also not usually 
apparent on the surface (e.g., Chara). Some such as Kelp will be visible at the 
lowest tides. I don't think any of these sensibly qualify for the current 
natural=wetland tag, which implicitly connotes emergent vegetation typically on 
a land surface which can dry out. Some emergent vegetation will have it's foot 
in the water, but water depth will be shallow (typical Phragmites beds). I'd 
suggest considering a new tag for submerged vegetation in marine environments.
At this point I'm not sure about freshwater vegetation which is totally 
submerged (but see below).
For the detailed US classification of wetlands, this is what the 
plant_community tag is there for: more detailed, more scientifically precise 
categories. Obviously many of these are not easy for the average mapper to 
identify, but when information is available it is often a good way of enhancing 
the base tagging. I think there is also a US National Vegetation 
Classification. I've documented some parts of the UK equivalent, including one 
of the types of Alder Carr (equivalent to your Alder meadow).
Phragmites does grow in estuarine environments, with brackish water.
I learnt some years ago that German usage of reedbed (Rohr) includes other tall 
emergent plants: notably sedges (Carex), Cladium, reed-mace/cat's-tails 
(Typha), cane (Arundo) and club-rushes. PresumabThus the meaning of 
wetland=reedbed may well be wider than expected in some countries. One way to 
be sure is to add a dominant_taxon tag (e.g., Phragmites australis, Carex, 
Typha etc).
As for floating water plants (which I would not particularly class as emergent, 
including water-lilies) they can have odd life cycles. Some living on the bed 
of the water body when dormant and later floating during the growth season 
(Water Soldier & Frogbit). There are certainly places in France where a carpet 
of Duckweed coats the waterways during the summer. Unlike Kelp & Eelgrass beds 
(one a significant carbon sink, the other increasingly threatened) I've never 
felt the need to map such things (they are seasonal, and usually quite small 
features). Even something like Water Hyacinth which does form large patches is 
likely to change because of control measures.

In Wednesday, 18 December 2019, 22:25:50 GMT, Kevin Kenny 
<kevin.b.ke...@gmail.com> wrote:  
 On Wed, Dec 18, 2019 at 2:08 PM Clifford Snow <cliff...@snowandsnow.us> wrote:
> How should eelgrass[1] be tagged? I see that wetland=reedbed [2]  has been 
> used in tidal areas mainly in Europe but also in the US but they are two 
> different plants.

Perhaps wetland_class=emergent or wetland_class=aquatic_bed? (How does
the eelgrass grow in the area you're considering?)

Thus saith 'Classification of Wetlands and Deepwater Habitats of the
United States' (https://www.fws.gov/wetlands/documents/classwet/index.html):

https://www.fws.gov/wetlands/documents/classwet/emergent.htm :
Definition. The Emergent Wetland Class is characterized by erect,
rooted, herbaceous hydrophytes, excluding mosses and lichens. This
vegetation is present for most of the growing season in most years.
These wetlands are usually dominated by perennial plants. All water
regimes are included except subtidal and irregularly exposed.

Definition. The Class Aquatic Bed includes wetlands and deepwater
habitats dominated by plants that grow principally on or below the
surface of the water for most of the growing season in most years.
Water regimes include subtidal, irregularly exposed, regularly
flooded, permanently flooded, intermittently exposed, semipermanently
flooded, and seasonally flooded.

Aquatic beds further divide into algal (e.g., kelp, rockweed,
stoneword), moss (e.g. Fisseidens, Fontinalis), rooted vascular
(Zostera would fall in this category), and floating vascular
(duckweed, water lettuce, water hyacinth, water-nut (Trapa), water
fern (Salvinia), bladderwort, and so on).

Rooted vascular aquatic beds occur in marine, estuarine, riverine,
lacustrine and palustrine systems

Some species, such as the water lily Nuphar luteum, are hard to
classify between 'aquatic bed' and 'emergent', since it usually grows
as lily pads, but occasionally stands erect above the water surface.
Some of the eelgrasses have the same difficulty in classifying.

The categories are always going to be fuzzy around the edges.

USFWS would therefore label your eelgrass bed - if I understand
correctly what you're trying to label - as "Marine, subtidal, aquatic
bed, rooted vascular"  while a typical reedbed might be "palustrine,
emergent wetland, persistent, dominant vegetation Phragmites spp." and
a typical alder meadow near me could be "palustrine, scrub-shrub
wetland, broad-leaved deciduous, predominant plant Alnus spp."

I have Absolutely No Idea how to fit a classification scheme like
Cowardin's into a 'folksonomy' like OSM's.

73 de ke9tv/2, Kevin

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