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Scientists flag unusual winter chlorophyll dip; nutrient mass‑balance and lab changes underway
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Summary
USGS and university researchers reported an unexplained January–February dip in water‑column chlorophyll, discussed nutrient‑limitation and microbialite/resuspension hypotheses, and said USGS will move hypersaline nutrient analyses to Chesapeake Biological Labs while preparing a nutrient mass‑balance draft for December.
USGS scientists and university researchers told the interagency meeting that an uncharacteristic suppression of measured chlorophyll occurred in January–February, with water‑column chlorophyll values near zero despite conditions that usually favor phytoplankton growth.
Presenters outlined three leading hypotheses: (1) reduced nutrient delivery to the sampled pelagic zone (either because inflows were low or nutrients were exported to the North Arm); (2) uptake or sequestration of nutrients by microbialites and biofilms that temporarily remove nutrients from the water column; and (3) physiological suppression of photosynthesis during cold periods even when chlorophyll pigments remain present. Participants pointed to coincident patterns in past years and to model results that show spikes in cyanobacteria and diatoms are often correlated with wind or cold events that can resuspend microbial mat material.
Gary Golowski summarized lab experiments showing brine‑fly larvae consume shrimp pellets and feces and increase decomposition rates (an experiment reported ~300% faster decomposition when larvae are present), implying brine flies can accelerate nutrient recycling and thus influence pelagic nutrient availability. Gary also noted that changes to the causeway/berm (modifications beginning in 2022) and other hydrologic shifts appear to have altered nutrient dynamics compared with pre‑2006 conditions; model fits to observed chlorophyll differ before and after those changes.
USGS staff reported that the North Arm is a dominant source of nutrient mass for the system in recent calculations and that cumulative streamflow and inflows were substantially below last year after June. They also described analytical issues with hypersaline samples at the National Water Quality Lab (refusal to run hypersaline nutrients and poor recoveries for some analytes). As a result, the program is transitioning routine hypersaline nutrient analyses to Chesapeake Biological Labs and will perform a formal lab‑comparison and document any step changes in the record.
Partners plan to publish a nutrient mass‑balance analysis and data products (a draft was reported under development) by the December meeting, and to investigate how short, stochastic wind or cold events can be incorporated into interpretation of monitoring data. Presenters emphasized that multiple mechanisms could operate together and that current hypotheses remain under active investigation.
