Using the 2.4-million-word written Spanish subsection of the Corpus of Utah Dual Language Immersion, collected from a large-scale standardized writing assessment of K-12 Spanish second language (L2) students, this study focuses on lexical complexity as operationalized by three measures: lexical diversity, lexical density, and lexical sophistication. The study goes beyond most previous work on lexical complexity by investigating the effect of task type on all three measures of lexical complexity. Patterns in variation are identified across proficiency levels and between task types. Results show that all three measures increase at each proficiency score between Novice High and Advanced, except at Intermediate Mid where scores dip or flatten. Diversity and sophistication are both shown to increase rapidly after this mid-point, indicating that a broad and deep lexical repertoire is a key feature of more advanced proficiency levels. Results for the effect of task indicate that text genre impacts learners' lexical density, while tasks that are more complex elicit higher lexical sophistication.