Spatial and temporal variability of internal and external phosphorus loads in Mona Lake, Michigan

Abstract

A study was conducted in Mona Lake, a small eutrophic lake located in western Michigan (USA) to address the temporal and spatial variability of external and internal phosphorus loading. External P load varied among subbasins, which was mostly related to discharge, but also to land use. Black Creek, which drains lands with natural cover and agriculture, accounted for the majority of flow, and total phosphorus (TP) and soluble reactive phosphorus (SRP) load, to Mona Lake. However, the relative contribution of SRP load was greater in Little Black Creek, which flows through a mostly urbanized subbasin, than in Black Creek. The relative importance of internal loading was strongly related to season, as internal TP loads contributed only *9% of the overall P load in April 2005, but *68-82% of the overall P load in the summer and early fall seasons. Internal TP and SRP loading was greater under anaerobic than aerobic conditions. Mean anaerobic TP release rates ranged from 0.80 to 15.56 mg P m -2 d -1 , varying with site and season. Spatial variability in both internal phosphorus loading and sediment P concentration was also evident. By taking into account the spatial and temporal variability of different loading sources, management practices can be targeted to optimize nutrient source control strategies.

Laboratory-determined Phosphorus Flux from Lake Sediments as a Measure of Internal Phosphorus Loading

“…Excess sediment can be carefully let out the bottom of the core tube by removing the bottom stopper; add water from the carboy collected at the corresponding site, if needed. Commonly-used sediment and water column depths are 20 cm of sediment with a 25 cm overlying water column 2,4,13,[17][18][19] , but these amounts can be modified as desired. 2.…”

“…Calculate flux (release rate) based on the change in water column TP or SRP using the following equation 2 : P rr = (C t -C 0 ) × V / A where P rr is the net P release (positive values) or retention (negative values) rate per unit surface area of sediment (mg P/m 2 /d), C t is the TP or SRP concentration in the water column at time t, C 0 is the TP or SRP concentration at time 0, V is the volume of water in the water column of the core tube, and A is the planar surface area of the sediment cores. Calculate P release rate using the linear portion of the concentration vs. time curve to give the maximum apparent release rate 4,13,18,19 . To avoid potential short-term bias, choose nonconsecutive sampling dates for C t and C 0 18,19 .…”

“…Historically, quantification of P loading to lakes focused on external sources, or P originating in the watershed via point and nonpoint sources. However, internal loading from lake sediments can account for a large portion, if not the majority, of the total P load in eutrophic lakes [2][3][4][5] . Thus, even substantial reductions in external loading to lakes can fail to result in water quality improvement due to the overriding effect of P release from sediments [5][6][7][8] .…”

“…Internal load estimates based on in situ measurements suffer from high variability associated with the inherent spatial and temporal variability of environmental data and can be affected by inadequate monitoring frequency situ and mass balance approaches. Laboratory incubations of sediment cores, combined with monitoring of external loads, have been used to determine the relative contributions of internal and external P loads, with the goal of guiding management decisions to optimize nutrient source control 2,4,17 . In two Michigan lakes with extensive shoreline development and high percentages of impervious surface (>25%) in the sub-basins directly adjacent to the lake, internal P load was estimated to account for up to 80% of the total P load, prompting recommendations to focus management efforts on reducing sediment P release 2,4 .…”

“…Laboratory incubations of sediment cores, combined with monitoring of external loads, have been used to determine the relative contributions of internal and external P loads, with the goal of guiding management decisions to optimize nutrient source control 2,4,17 . In two Michigan lakes with extensive shoreline development and high percentages of impervious surface (>25%) in the sub-basins directly adjacent to the lake, internal P load was estimated to account for up to 80% of the total P load, prompting recommendations to focus management efforts on reducing sediment P release 2,4 . In contrast, experimental studies of sediment from a less developed lake in the same region showed that internal loading composed only 7% of the total P load, prompting a recommendation to focus P management strategies in the watershed 17 .…”