Persistence and intervening states of warm low oxygen episodes at eight southeastern United States estuarine stations

This study describes the temporal organization of simultaneous warmth and low oxygen in eight fixed nearbottom estuarine station records.Original 15-minute temperature-oxygen observations from May-September 2018-2023 supplied 35 eligible station-seasons and 510,454 valid pairs (99.29% of scheduled observations within those seasons).Warmth exceeded a fixed station-specific P90 of valid annual temperatures during 2018-2023; low oxygen was below 2 mg/L.Four observed states distinguished neither condition, warmth only, low oxygen only, and their overlap.The records contained 417 compound episodes and 412 nominal occupancy hours.Episode count and the fraction of occupancy assigned to sustained runs described different aspects of persistence.Among 384 fully observed interepisode intervals, 207 lacked a neither-state reading.A post hoc partition identified 183 entirely warm-only intervals, 22 entirely low-oxygen-only intervals, and two containing both single-condition states.These intervals represented 53.9% of the count but only 7.1% of total interval hours.Their count fraction fell to 40.9% under stricter coverage and 43.7% under conservative deployment screening.An exploratory two-common-year station comparison was sensitive to P90 versus P95 and does not establish a persistent station difference.The contribution is a descriptive four-state characterization of monitored sequences, separating episode counts, occupancy-weighted persistence, and intervening conditions.It does not identify ecological mechanisms or organismal responses.