Connectivity Influences Temporal Variation of Limnological Conditions in Missouri River Scour Lakes

ABSTRACT

Two scour lakes in the Missouri River floodplain were sampled 20 months (April 1996 – December 1997) to examine effects of river inflow on mixing, seston, and nutrient dynamics. Lake NC-11 (≈4 ha) is deep (≈16 m), sheltered, dimictic, and connected to the river only during floods. Lake S-19 (≈20 ha) comprises two sub-basins, a deep (≈12 m) riverine forewater perennially connected to the river and a shallow (≈4 m), polymictic backwater of variable connectivity. Stratification in NC-11 affected, and was affected by, river inputs – cool inflows mixed completely and sometimes destratified the lake, warmer inflows were partly or completely confined to the epilimnion. Mixing between basins at S-19 was incomplete except during floods. During floods, concentrations of seston and particulate N and P were similar in the two basins of S-19, but conditions in the two basins diverged substantially during inter-flood periods as lower water levels reduced lateral exchange. Seston concentrations in NC-11 during floods were much less (≈90% less) than in S-19 or the river. In NC-11, algal blooms (chlorophyll 50–90 μg L⁻¹) occurred in summer post-flood periods, but larger blooms (chlorophyll 50–256 μg L⁻¹) occurred in the S-19 backwater during periods of low connectivity in all seasons. Nitrate-N peaked at >2 mg L⁻¹ during floods but was exhausted during algal blooms along with up to ≈90% of dissolved phosphorus. Silica declined sharply (up to 93%) in many, but not all blooms, but remained >0.5 mg L⁻¹ in all observations. Connectivity influenced many lake features and enhanced nutrient availability, but other floodplain lakes lacking river connections have similar algal biomass.

Key Words:

• Missouri River
• floodplain lakes
• scour basins
• nutrients
• chlorophyll
• suspended solids
• connectivity
• 1993 flood