The horizontal and temporal distribution of hydrographic properties and phytoplankton biomass along the Catalan coast were analyzed to determine their variance structure (i.e., variance partition between temporal, alongshore, and across-shore components), and the relationship between hydrographic heterogeneity and variability in phytoplankton biomass. Temporal chlorophyll variability was highly seasonal, but horizontal biomass variability was related to hydrographic variability, as evidenced by significant correlations to temperature and salinity. The strength of chlorophyll-salinity correlations changed according to the extent of riverine influences within the area, but significant correlations were always negative, indicating enhanced phytoplantkon growth due to river runoff. Temperature-chlorophyll correlations changed seasonally, from highly positive in early summer to negative by late summer. In addition, horizontal biomass patterns were dominated by an onshore-offshore decrease, which were altered by modifications of water circulation associated to topographic features such as canyons and shelf concavities.