The Maryland Biological Stream Survey (MBSS) was started by the Maryland Department of Natural Resources in 1993 as a small pilot study in three watersheds. A second, larger demonstration project, expanded statewide, was conducted in 1994. The MBSS was Maryland's first probability-based or random design stream sampling program intended to provide unbiased estimates of stream conditions with known precision at various spatial scales ranging from large 6-digit river basins and medium-sized 8-digit watersheds to the entire state. The basis of the MBSS design is lattice or multi-stratification sampling that ensures all 1st through 3rd order (now 1st through 4th order), non-tidal streams in the sampling frame have a non-zero and known probability of being sampled. A stratified random design is a cost-effective way to characterize Maryland's 10,000+ miles of freshwater streams.
The first statewide round of the MBSS was completed in three years. The survey was based on a random selection of stream sites located within each 6-digit river basin. An approximately equal number of sites were selected from each of the three stream orders (1st, 2nd, 3rd) in each basin. This method weighted 3rd order streams most heavily (to assess fishability), because the miles of 3rd order streams are smaller (8.6% of the total miles on a 1:250,000 scale map), with progressively more 2nd order (18.7% of the total) and 1st order (72.7% of the total) streams. 100% of the Round One sites were randomly chosen.
The second statewide round of the MBSS was completed in five years. Round Two sampling design was modified to include both a core component of randomly-selected stream sites plus targeted sampling effort dedicated to special studies. To meet the State's growing need for information on stream conditions at finer spatial scales, the Round Two core random component was re-designed to focus on Maryland's 8-digit watersheds, using a new 1:100,000 scale map. More small streams were sampled by the MBSS during Round Two compared to Round One. All 1st through 4th order, non-tidal streams on the 1:100,000 map still had a non-zero and known probability of being sampled, as in Round One. However, unlike Round One, randomly-selected stream sites for Round Two were selected from all four stream orders in proportion to their number of stream miles. In addition, 27 minimally-disturbed stream sites were selected in 2000 for a new Sentinel Site Network, where sites were sampled every year with MBSS protocols to aid in assessing natural year-to-year variation in stream conditions. In addition, several other targeted sites were sampled for special studies. Therefore, by 2004, 74% of the Round Two MBSS sites were randomly located and 26% were targeted.
When the five-year Round Three started in 2005, the MBSS sampling design was continuing to evolve. The core, stratified random component and the Sentinel Site Network were still in place. But, effort had shifted to even more targeted sampling sites to continue the Sentinel Site Network and achieve several special study objectives. In 2008, the MBSS sampled almost 290 sites in 1st through 4th order, non-tidal streams across the state. Only 84 sites (29% of the total) were randomly selected. The other 70% were targeted sites selected for these special studies: 66 sites supported MDE's TMDL and Tier II watershed delineation needs, 26 sentinel sites, 25 sites in Stronghold Watersheds, 6 sites in the Corsica River watershed, 6 sites focused on imperiled freshwater mussel habitats, plus several sites in potential Maryland darter and acuminate crayfish habitats.
The allocation of sampling effort in 2008 is representative of what the MBSS will look like in the foreseeable future. This 'new' MBSS is currently much more than only a stratified-random monitoring program. The survey has evolved, but still retains its original objectives: to collect data from randomly-located sites, to calculate unbiased estimates of stream conditions with known precision, and to assess the impacts of acid deposition on aquatic biota at various spatial scales for status and trends analyses. But, in addition, effort allocation has shifted to include much more targeted sampling focused on a wide range of other program objectives. Fifteen years after its beginning, the MBSS is now a more balanced stream monitoring program with random and targeted sampling components.
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