To convert biovolume of zooplankton to weight use assumption of specific gravity of 1 (like water) than: 1 million micron cube is equale to one (1) microgram.s

Yes, for wet weight ok, but for dry weight and ash content?

For dw and ash content you need ultramicrobalance

In any text on zooplankton methods it is possible to see different conversion factors (BV to dry mass, AFDM, organic carbon, etc.). It depends also on the method used to estimate biovolume. See also Alcaraz, M., Saiz, E., Calbet, A,. Trepat, I., Broglio, I. (2003). Mar. Biol. 143: 307-315.

In Method in zooplankton ecology (I think 1983, Omori and Ikeda), and in ICES Zooplankton Methods, edited by Harris et al., (2000) you have everything. Just read it.

Read this article

High Ash Content in Net-Plankton Samples from Shallow Coastal Water: Possible Source of Error in Dry Weight Measurement of Zooplankton Biomass NORIO NAGAO1, TATSUKI TODA1, KAZUTAKA TAKAHASHI2 *, KOJI HAMASAKI3, TOMOHIKO KIKUCHI4 and SATORU TAGUCHI1

Ash content for adult freshwater copepods is species specific and is around 2.4-7.3% of dry weight, see table 2.1 in Błędzki, L. A. and J. I. Rybak (2016). Freshwater Crustacean Zooplankton of Europe: Cladocera & Copepoda (Calanoida, Cyclopoida) Key to species identification, with notes on ecology, distribution, methods and introduction to data analysis, Springer