Which physicochemical variables should zooplankton ecologists measure when they conduct field studies?

Over the last century, a rich literature has developed describing how the physical and chemical environment influences zooplankton communities, but there is little guidance on the suite of limnological variables that should be measured by zooplankton ecologists. We performed a literature review to assess: 1) Which variables were measured most often by zooplankton ecologists; 2) Which of these variables were consistently related to zooplankton abundance and richness; and 3) Whether key variables were overlooked by investigators. Our results show that there is a core group of nine limnological variables that are measured most frequently, including lake surface area, pH, phosphorus, nitrogen, dissolved oxygen, conductivity, chlorophyll-a, maximum depth, and temperature. These variables were among those most often associated with variation in zooplankton, but several others, including dissolved organic carbon, alkalinity, and nitrate were sampled infrequently, despite showing promise as important explanatory variables. The selection of variables in past studies did not correlate with how often those variables were significant in the literature, but instead, might have been related to their ease of measurement. Neglecting to measure important variables could have implications for fundamental and applied studies that aim to understand the factors structuring zooplankton communities and their response to environmental change.

Methods
We performed a literature review to identify studies that investigated the environmental variables associated with zooplankton community structure in lakes or ponds using Google Scholar. For the purposes of our study, we defined community structure as either univariate metrics used to describe communities (richness, diversity, total abundance) or multivariate analyses such as principal component analysis, which summarizes relative species abundance. We considered studies published between 1970 to 2019 and used the following search terms to identify relevant studies 1) zooplankton distribution and abundance; 2) zooplankton abundance environmental factors; 3) zooplankton diversity environmental factors; 4) zooplankton richness environmental factors; and 5) zooplankton lakes. Studies that collected data from fewer than three lakes or that included lakes with a salinity greater than 5 ‰ were excluded from our dataset. For each study, we extracted information on which physicochemical variables were measured and whether the investigators found a significant relationship between zooplankton communities and these variables. Where available, we also obtained the proportion of variation in zooplankton community structure explained by each variable in a study, as well as the mean, minimum, and maximum values measured for each environmental variable. When necessary, units of measurement were converted to ensure that all data were analyzed on the same scale.