Freshwater ecosystems are a good model for studying questions associated with environmental change because their nutrients and biota reflect changes in their surrounding watershed. Although many studies of fresh waters focus on understanding how the terrestrial landscape affects lake, stream, and wetland properties, it is becoming more widely recognized that we must also consider the freshwater landscape, including connections (or lack thereof) among surface waters, to understand and manage freshwater ecosystems. However, freshwater types continue to be studied individually (e.g., lakes or streams or wetlands) or at relatively local scales (e.g., an individual lake to tens of lakes within a watershed), whereas environmental changes such as land use intensification, climate change, and the spread of non-native species affect all freshwater types and often occur at broad spatial scales such as regions, continents, and the globe. Understanding patterns of freshwater properties at broad-extents can be complicated by the influence of drivers operating at different spatial scales (i.e. cross-scale interactions). An emergent specialty in ecology, macrosystems ecology, provides a framework for broad-extent, multi-scale, and cross-freshwater type studies, improving predictions and contributing to the understanding of freshwater ecosystem responses to change at the regional to continental extents relevant for management and policy. In addition, the recent rise of open science perspectives and advancements in computational tools makes it possible to collate data and perform analysis at the macroscale. My dissertation uses this theoretical foundation and publicly-available databases to understand macroscale patterns in nutrients and fish biodiversity across freshwater types and the processes that may underlie those patterns.In my first chapter, I compiled total phosphorus, total nitrogen, chlorophyll a, and percent macrophyte cover from over 3,500 lakes, streams, and wetlands sampled by the Environmental Protection Agency’s National Aquatic Resource Surveys across the continental U.S. This research led to the understanding that these different freshwater types may share similar patterns and drivers of nutrients across the U.S., but different patterns and drivers for biotic properties. For my second chapter, I further investigated biotic properties across lakes and streams, specifically focusing on fish biodiversity patterns and connections between lakes and streams. I used fish data from 559 lakes and 854 streams from the midwestern/northeastern U.S. and found that discrete connectivity classes helped explain variation in fish species composition and richness across lakes and streams. My third chapter is a data paper that describes methods for creating a database (LAGOS-NETWORKS) that includes a suite of surface connectivity metrics for 86,511 lakes and 898 networks in the U.S. This is the first database to provide accessible and comprehensive lake network metrics at the national scale. LAGOS-NETWORKS was used in my final chapter, where I used these continuous connectivity metrics and other multi-scale drivers to investigate how the effects of connectivity on fish species richness change with regional-scale land use. I found that connectivity had different effects on lake and stream fish species richness depending on regional-scale agricultural land use, showing a cross-scale interaction (CSI) and the effect of this CSI differed by freshwater type. Collectively, my dissertation uses multi-scale, cross-scale, and an integrated freshwater landscape approach to further understand patterns and processes in aquatic ecosystems at the macroscale.
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