Combining low- and high-pressure membranes into an integrated membrane system is an effective treatment strategy for seawater desalination. Low-pressure microfiltration (MF) and ultrafiltration (UF) membranes remove particulate material, colloids, and high-molecular-weight organics leaving a relatively foulant-free salt solution for treatment by high-pressure reverse osmosis (RO). An integrated membrane system is severely challenged, however, when an algal blooms occurs. This research investigated important factors for and mechanisms of fouling by bloom-forming algae in integrated membrane systems. In order to study RO fouling by algae, bench-scale testing protocols were evaluated. A method was developed to measure concentration polarization and intrinsic membrane permeability in a single experimental test. This laid the groundwork for accurately measuring flux decline from fouling. Natural seawaters (not under the influence of algal blooms) did not cause flux decline through buildup of organic foulants even though organic matter did deposit on the membranes. Significant deposition of algae and algogenic organic matter (AOM) was observed on RO membranes. Dissolved AOM caused little flux decline, but when particulate material was present the flux declined by ten percent over 24 hours. The organic-matter cake layers that formed on the membrane surface caused some resistance to flow, but it was small compared to the intrinsic membrane resistance. Modeling showed an interplay between two fouling mechanisms: cake-enhanced concentration polarization (CECP) and hydraulic resistance. For foulant cakes with low porosity and small particle size, hydraulic resistance may be more important than CECP. Foulant deposition on RO membranes was greatly affected by the feed spacer but was also affected by the shape of the permeate carrier. A permeate carrier with a fine mesh resulted in more spatially homogeneous foulant deposition and lower fouling propensity from AOM. A coarse permeate carrier caused foulants to deposit in a pattern defined by the feed spacer and flux decline was exacerbated. It is suggested here that both the feed spacer and the permeate carrier are important in determining the hydrodynamics that lead to foulant deposition. Unlike RO membranes, low-pressure microfiltration (MF) and ultrafiltration (UF) membranes were severely fouled by algae. Pore blocking was the dominant mechanism for the early stages of MF and cake filtration dominated during later stages as irreversibly blocked pores became unavailable. Cake filtration was the dominant mechanism in UF where the pores were smaller and foulants were excluded. Hydrodynamic shear applied to algal cells was detrimental to low-pressure filtration because of the release of highly-fouling organic matter. The highly fouling fraction was material large enough to be retained on a 0.22-micrometer microfilter but small enough that it was not removed by centrifugation or coarse (glass fiber) filtration. The algal cells themselves did not fall within this fraction since they were easily removed by centrifugation, but particulate material derived from broken algae did fall into the highly fouling fraction. Bacteria were also included in the highly-fouling fraction and were shown to be a major factor in fouling during an algal bloom. Adsorption of dissolved AOM was not a significant fouling mechanism on the hydrophilic membranes used here that are similar to the most common full-scale low-pressure membranes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]