Aquatic ecologist david strayer of the institute of ecosystem studies and a team of researchers from new york state's department of ecological conservation (dec) took an in-depth look at populations in the hudson river, which zebra mussels had invaded in 1991. using data from before and after the zebra mussel invasion, strayer and his team observed different trends in plankton and invertebrate populations in the river's open water versus the shallow water in the littoral zone near the shore. there is a scheme representing a riverbank and water near it. in water there is a horizontal line dividing it into open water zone and deep water zone. open water zone area which is closer to the riverbank and where there is no deep water zone is labeled littoral zone. in the open water, zooplankton and invertebrate populations declined by 70% after the zebra mussel invasion. in the littoral zone, bottom-dwelling invertebrate populations increased after the zebra mussel invasion. this was likely due to the mussel shells providing habitat structure and hiding places for these creatures on the river bottom, and also because the feces of zebra mussels enriched the river bottom with nutrients. also, the improved water clarity that results from mussel feeding enables more light to reach the bottom. this promotes the growth of underwater plants, which provide food and shelter for littoral invertebrates. these contrasting trends in the open deep water and the shallows of the littoral zone led strayer's team to propose hypotheses and predictions about fish populations in the hudson. drag the labels to illustrate these steps of the scientific process. labels may be used more than once.