Phytoplankton, including microalgae and cyanobacteria, are important players in global ecosystems, shaping the balance of all life forms on earth. Recently, aquatic ecosystems have been disrupted by climate change, which leads to more frequent occurrence of harmful algal blooms (HABs). HABs are characterized by the sudden growth of photosynthetic algal cells in both fresh and marine water, with some blooming species producing harmful toxins. The occurrence of HABs endangers water resources for drinking, fishing, and recreation, leading to huge ecological and economical costs. Despite the urgency of the problem, the mechanistic understanding of how complex environmental conditions trigger HABs is still not well understood. This is in part due to the lack of high throughput tools for screening environmental parameters that promote the growth of photosynthetic microorganisms. This dissertation focuses on the development of a microfluidic platform to model the growth of phytoplankton cells under a controlled microenvironment. The unique aspect of the microfluidic platform is its ability to provide well-defined chemical gradients as well as physical (light) gradient for studies of algal cell growth. The experimental data is amendable to theoretical modelling. An array microhabitat device with well-defined single and dual chemical gradients was developed to study the effects of nutrients, and a microscope-based light gradient generator was developed to study the effects of light intensities on algal growth. This platform provided 64 different environmental conditions at the same time, with the chemical gradient generation taking less than 90 min, and the light gradient easily adjustable with lamp voltage and a customized mask. Using the platform, it was revealed that nitrogen, phosphorous, as well as light, synergistically promote the growth of unicellular model microalga Chlamydomonas reinhardtii. The growth response to single and dual environmental gradients was fitted well with Monod growth kinetic models. Besides physical and chemical factors, biological interactions in the microenvironment of phytoplankton were reviewed, and a preliminary coculture study showed growth suppression of C. reinhardtii by a bloom forming cyanobacterium, Microcystis aeruginosa. This work highlighted the enabling capability of the microfluidic approach for mechanistic understanding the effects of multiple environmental factors on phytoplankton cell growth.