That looks nice, two comments:

1. I prefer bgc = PISCES(grid). The interface is a bit split whether grid should be an arg vs kwarg but I think we have been gradually drifting towards grid as positional arg (the big change is for models, which will have to come later).

2. There's a discussion about making it easier to change number type here: How can we help users change number type more easily? CliMA/Oceananigans.jl#3800

(not worth it to do the sugar kelp model because it has a billion parameters)

I don't follow this statement --- what do you mean?

For situations with large numbers of parameters, you can put them in an Array (like TabluatedAlbedo for ClimaOcean):

https://github.com/CliMA/ClimaOcean.jl/blob/fc175f6c6b3e095bd460d10d323e4361006a5008/src/OceanSeaIceModels/CrossRealmFluxes/tabulated_albedo.jl#L90

Another possibility is to convert at runtime, eg with something like

const parameter = 1.23

f(a::FT) = a + convert(FT, parameter)

Sorry I missed this I've been a bit busy.

I think that makes sense, there are a lot of instances here where (; grid) should probably be (grid; ) so I'll make a PR.

On the sugar kelp model, I didn't do it because I was being lazy but I'll change them before I merge the PR.

On the sugar kelp model, I didn't do it because I was being lazy but I'll change them before I merge the PR.

You can also make a note in the source code and then open an issue about improvements if it doesn't make sense to prioritize immediately

Maybe I'm misunderstanding the issue but with tons of parameters can't we loop over each property and convert it?

Something like this:

function Base.convert(::Type{SugarKelp{FT_new, TL}}, x::SugarKelp{FT_old, TL}) where {FT_new, FT_old, TL}
    fields = NamedTuple{fieldnames(typeof(x))}(
        field === :temperature_limit ? getfield(x, field) : convert(FT_new, getfield(x, field))
        for field in fieldnames(typeof(x))
    )

    return SugarKelp{FT_new, TL}(; fields...)
end

Oh yes that's a good idea, I hadn't thought of that.

Do you think we would then need to put in the Biogeochemistry constructor a convert(FT, particles) (which would mean we always have to have convert defined, or do you think it would be fine to leave this to the user like:

particles = SugarKelp(...)

particles = convert(FT, particles)

biogeochemistry = Biogeochemistry(...; particles)

do you think it would be fine to leave this to the user like:

You want to throw an error if the float type is wrong