We present results from a series of tracer dye experiments where ocean plume concentration is measured using unmanned surface vessels. The goal of the study is to characterize the fine-scale spatiotemporal plume structure which is used for developing and evaluating autonomous robotic sampling strategies. We present a description of the qualitative characteristics of experimentally generated marine plumes namely intermittency, sinuous structure, and the time varying near-source concentration profile. We also present a data reduction process and a set of summary statistics to describe the fine-scale plume structure as evidenced by the time series measurements. These summary statistics provide a comparison benchmark for the development of future plume simulation models that capture the fine-scale plume structure.