In a change from previous SAV distribution and abundance reports in this series, SAV distribution data in this report are presented and discussed based on the segmentation scheme adopted by the Chesapeake Bay Program (U.S. EPA, 1983) and described briefly in the Methods section of this report. A new zonation scheme (Upper, Middle, and Lower zones) for Chesapeake Bay, which accommodates the Chesapeake Bay Program segmentation boundaries, was adapted from that used in previous SAV distribution reports (i.e., Orth et al., 1994) and established by Orth and Moore (1982), then modified by Orth et al., (1989). SAV distribution data for the years 1991-1994 are presented using the Chesapeake Bay Program segmentation scheme in order to consistently follow up the recently published EPA report on trends in SAV (Orth et al., 1995), which covers the years 1971-1991 using the same scheme.
The 1991-1993 data were edgematched using ARC/INFO GIS software, as were the 1994 data, in order to bring separately digitized USGS 7.5 minute topographic quadrangle SAV coverages into one unified coverage for the entire Chesapeake Bay (see Methods). Therefore, 1991-1993 SAV distribution data presented in this report reflect edgematching adjustments, and differ from previously published data for these years derived from separate coverages which were not edgematched (i.e., Orth et al., 1992, 1993, and 1994).
In 1994, the Chesapeake Bay had 26,484 hectares of SAV, compared with 29,587 hectares in 1993, a decrease of 10%, with 3,854 hectares (15%), 13,373 hectares (50%), and 9,257 hectares (35%) occurring in the Upper, Middle, and Lower Bay zones, respectively (Figures 1 and 2). SAV increased in the Upper zone by 44%, and decreased in the Middle and Lower zones 21% and 8%, respectively, from totals in 1993 (Figure 2). SAV increased in abundance from 1993 levels in all segments in the Upper Bay zone except WT1 (Bush River) and WT4 (Back River), which remained unvegetated. Increases in 1994 for some segments (e.g., TF1 and CB8) of the Middle and Lower zones did not offset larger decreases in 1994 in other segments (e.g., CB5, TF2, EE3, CB7, and LE3) of these zones.
In 1994, SAV in the Bay increased, compared with 1993 levels, in twenty segments (Upper zone: CB1, CB2, CB3, WT2, WT3, WT5, WT6, ET1, ET2, ET3, ET4; Middle zone: WT7, WT8, TF1, RET1, LE2, EE1; and Lower zone: CB8, LE4, LE5), decreased in fourteen (Middle zone: CB4, CB5, LE1, TF2, RET2, ET5, ET8, ET9, EE2, EE3; and Lower zone: CB6, CB7, LE3, WE4), and remained unvegetated in eleven (Upper zone: WT1, WT4; Middle zone: ET6, ET7, ET10; and Lower zone: TF3, RET3, TF4, RET4, TF5, RET5). SAV was not detected in 1994 in 14 segments (Upper zone: WT1, WT4; Middle zone: CB4, LE1, ET5, ET6, ET7, ET10; and Lower zone: TF3, RET3, TF4, RET4, TF5, RET5) (Figures 3, 4, and 5) as compared to 16 segments in 1993. [The mainstem upper and middle Rappahannock River, TF3 and RET3, upper and middle York River, TF4 and RET4, and upper and middle James River, TF5 and RET5, have been totally devoid of submerged vegetation for years and are not currently photographed (Methods)]. Five segments (Upper zone: WT5, ET1; and Middle zone: WT7, WT8, RET1), which were unvegetated in 1993, had some SAV detected by photography in 1994 (Figures 3 and 4). Three segments (CB4, LE1, and ET5), all in the Middle zone, which were vegetated in 1993, albeit with relatively small amounts, had no SAV detected in 1994 (Figure 4).
In the Bay in 1994, 36% of SAV was categorized as dense (density class 4, or 70-100% coverage), up slightly from the 1993 amount (32%). The percentage of SAV categorized as moderate (density class 3, or 40-70% coverage) in 1994 (19%) also increased slightly from that in 1993 (18%). The percentage of SAV categorized as sparse (density class 2, or 10-40% coverage) (28%) decreased in 1994 compared with that in 1993 (34%), whereas that categorized as very sparse (density class 1, or 1-10% coverage) (17%) increased slightly in 1994 from that in 1993 (15%).
SAV in the very sparse and sparse classes combined (the 0-40% coverage range) constituted 45% of all SAV in 1994, a decrease from 49% in 1993. Conversely, SAV in the moderate and dense classes combined (the 40-100% coverage range) constituted 55% of all SAV in 1994, an increase from 51% in 1993.
The total SAV in Chesapeake Bay increased each year from 1991 to 1993, then decreased in 1994. (Figure 2). However, the 1994 level of 26,484 hectares was still 859 hectares greater than the 1991 level of 25,625 hectares. SAV in the Chesapeake Bay increased in all zones each year from 1991 to 1993 (Figure 2). However, the 1994 level of 26,484 ), although not all segments within zones exhibited a steadily increasing trend. However, yearly gains in some segments offset losses in others, generally, resulting in increasing zone totals from 1991 through 1993. Then in 1994, the Upper Bay zone total continued to increase (44%) from 2,672 hectares the previous year to 3,854 hectares, the highest level in four years and an increase of 1,696 hectares over 1991 (Figure 2). However, in 1994 the Middle and Lower zone totals both decreased from 1993 levels, 16,825 to 13,373 hectares (21%) and 10,091 to 9,257 hectares (8%), respectively (Figure 2). The 1994 Middle zone total of 13,373 hectares was the lowest level in four years (a decrease of 921 hectares from 1991; a decrease of 3,452 hectares from the same period's highest level in 1993) (Figure 2). The 1994 Lower zone total of 9,257 hectares, although an 834 hectare decrease from its high for this four-year period in 1993, was still 84 hectares greater than the 1991 level (Figure 2).
In the Bay in 1994, SAV increased over 1991 levels in twenty-three segments (Upper zone: CB1, CB2, CB3, WT2, WT3, WT5, WT6, ET1, ET3, ET4; Middle zone: WT7, WT8, TF1, RET1, LE2, EE1, EE2; and Lower zone: CB6, CB7, CB8, LE4, WE4, LE5), decreased in nine segments (Upper zone: ET2; Middle zone: CB4, CB5, TF2, RET2, ET8, ET9, EE3; and Lower zone: LE3), and remained unvegetated in thirteen segments (Upper zone: WT1, WT4; Middle zone: LE1, ET5, ET6, ET7, ET10; and Lower zone: TF3, RET3, TF4, RET4, TF5, RET5).
In Chesapeake Bay, taken as a whole over the period 1991 to 1994, the percentage of SAV categorized as dense decreased 28%. In 1991, 50% of SAV in the Bay was in density class 4, but in 1994 this percentage was only 36%. The percentage of SAV in this class declined each year from 1991 until 1994, when it increased to 36%, its second highest level in the four-year period. That is a 13% increase from the four-year low in 1993 of 32%. Conversely, in the same four-year period, the percentage of SAV categorized as sparse increased 47%. In 1991, 19% of SAV in the Bay was in density class 2, but by 1994 this percentage was 28%. The percentage of SAV in this class increased each year from 1991 until 1994, when it decreased to 28%, its third highest level in four years. That is an 18% decrease from the four-year high of 34% in 1993.
The percentages of SAV in density classes 1 and 3 also increased from 1991 to 1994, although these increases were not as great as for density class 2. The percentage of SAV in density class 1 increased each year from 1991 to 1994. In 1991, 12% of Bay SAV was categorized as very sparse, but by 1994 this figure was 17%. The percentage of SAV in density class 3 in 1991 was 18%, in 1992 it was 26%, in 1993 it was 18%, and in 1994 it was 19%.
In the Bay from 1991 to 1994, there was a 45% increase in the percentage of SAV in the sparse and very sparse classes combined (the 0-40% coverage range). The percentage of SAV in combined classes 1 and 2 increased from 31% in 1991, to 49% in 1993, but in 1994 there was a decrease to 45% of the SAV. Conversely, there was a 20% decrease in the percentage of SAV in the moderate to dense classes combined (the 40-100% coverage range) from 1991 to 1994. The percentage of SAV in combined classes 3 and 4 decreased from 69% in 1991, to 51% in 1993, then increased in 1994 to 55%.
The total SAV coverage in the Upper Bay zone increased 44% from 1993 (2,672 hectares) to 1994 (3,854 hectares) (Figure 2). The total SAV level in the Upper Bay zone increased each year from 1991 to 1994, for an overall 79% (1,696 hectares) increase (Figure 2).
SAV in the Upper Bay zone, taken as a percentage of the SAV in the whole Chesapeake Bay, increased each year since 1991 (Figure 2). It increased less than a percentage point each year from 1991 to 1993, going from 8% to 9%; then in 1994 it jumped to 15% of SAV in the Bay (Figure 2).
Of the thirteen segments comprising this zone, there were increases in SAV abundance over 1993 levels in eleven segments, of which ten had increases in SAV over 1991 levels, also. Of the latter ten segments, eight segments (CB1, CB2, CB3, WT5, WT6, ET1, ET3, and ET4) were at their highest levels since 1991, and two of those segments, the Patapsco River (WT5) and the Northeast River (ET1), had SAV for the first time in this four-year period (Figure 3). The remaining two segments, the Gunpowder River (WT2) and the Middle River (WT3) reached their highest levels for this four-year period in 1992, then decreased in 1993, and rebounded in 1994 to their second highest levels since 1991.
Only one segment (ET2) had less SAV (180 hectares) in 1994 than in 1991 (269 hectares), although this was an increase from its lowest level of 95 hectares in 1993. SAV in 1994 in segment ET2, the Elk and Bohemia Rivers, increased 89% over 1993 (95 to 180 hectares), but this was still less than the four-year high for the period in 1991 of 269 hectares. Two segments, the Bush River (WT1) and the Back River (WT4) were unvegetated in 1994 and 1991. However, WT1 had some vegetation (2.32 hectares) mapped in 1992; whereas, only WT4 had no vegetation mapped in any year for the entire 1991 to 1994 period.
The amount of SAV in the Middle Bay zone decreased 21%, or 3,452 hectares, from 1993, to a total of 13,373 hectares in 1994, which was the lowest level in four years (Figure 2). The SAV in the Middle Bay zone increased each year from 1991 to 1993, for an overall increase of 18% (2,531 hectares), but the 1994 level was 921 hectares or 6% less than the 1991 level (Figure 2).
SAV in the Middle Bay zone, taken as a percentage of the total SAV in the Chesapeake Bay, declined in 1994 from 1991-1993 levels. In 1991 it was 56% of SAV in the Bay; in 1992 it was 58%; in 1993, 57%; and in 1994 it dropped over 6 percentage points to 50%. [This corresponded to concomitant increases in the other two zones.]
Of the 19 segments comprising this zone, there were increases in 1994 SAV abundance over 1993 levels in six segments (WT7, WT8, TF1, RET1, LE2, and EE1), which were also at their highest levels for the four-year period as well. In addition, three of the latter segments, the Severn River (WT7), the South, Rhode, and West rivers (WT8), and the Middle Patuxent River (RET1), had SAV for the first time in the four-year period (Figure 4). The Upper Patuxent River segment (TF1) was unvegetated in 1991 and 1992 as well, but had 8.78 hectares in 1993.
In the Middle Bay zone in 1994, there were decreases from 1993 levels in ten segments (CB4, CB5, LE1, TF2, RET2, ET5, ET8, ET9, EE2, and EE3). Six of the latter segments (CB4, LE1, ET8, ET9, EE2, and EE3) in 1993 were at their highest levels since 1991. Except segment EE2, the Lower Choptank River, which was still 1,402 hectares higher in 1994 than in 1991, nine of the segments which decreased from 1993 levels reached their lowest levels for this four-year period in 1994. This included, among others, the Upper and Middle Potomac River segments (TF2 and RET2) and the two segments (CB5 and EE3) encompassing Tangier, Smith, and Bloodsworth islands, which historically had some of the largest SAV beds in the Bay and, proportionately, the greatest amount of SAV in the Bay.
In the Middle Bay zone in 1994, there were six unvegetated segments (CB4, LE1, ET5, ET6, ET7, and ET10) (Figure 4), the latter three of which were unvegetated each year since 1991. The Lower Patuxent River segment (LE1) and the Choptank River (ET5) dropped back to their 1991 unvegetated condition after both had small amounts of SAV recorded in the intervening period. The Middle Central Chesapeake Bay segment, CB4, was vegetated in 1991, as well as in 1992 and 1993.
The total SAV level in the Lower Bay zone decreased 8% or 834 hectares from 1993 to 1994 (Figure 2). The total SAV level in the Lower Bay zone increased each year, from 1991 to 1993, for an overall increase of 918 hectares or 10% (Figure 2). SAV then decreased in 1994, but this level (9,257 hectares) was still 85 hectares greater than the 1991 level (9,173 hectares) (Figure 2).
SAV in the Lower Bay Zone, as a percentage of the SAV in the entire Chesapeake Bay, declined from the 1991 level of 36%, to 35% in 1994. It dropped to 33% in 1992, then increased in 1993, to 34%, and again in 1994.
Of the thirteen segments comprising this zone, there were increases in 1994 in SAV abundance over 1993 levels in three segments (CB8, LE4, and LE5), which also were at their highest levels in the four years since 1991. In 1994, four segments (CB6, CB7, LE3, and WE4) in the Lower Bay zone, which were consistently increasing each year since 1991, decreased from 1993 levels, which were the highest for the four-year period. In this zone in 1994, only one segment, LE3, the Lower Rappahannock River, decreased to a level (197 hectares) below that of 1991 (315 hectares).
In the Lower Bay zone, six segments (TF3, RET3, TF4, RET4, TF5, and RET5) which comprise the upper and middle segments of three major bay tributaries, the Rappahannock, the York, and the James, remained unvegetated from 1991 to 1994 (Figure 5). These six segments have been unvegetated for years and are no longer photographed (see Methods).
In 1994, SAV levels for six segments (CB6, CB7, CB8, LE4, WE4, and LE5) were higher than their 1991 levels, although three of these segments (CB6, CB7, and WE4) actually decreased from their 1993 levels, which were the highest for the four-year period.
SAV in the Chincoteague Bay section increased in distribution with 4,117.53 hectares mapped in 1994 compared with 3,576.57 hectares in 1993 (Figures 1 and 2). Most of the SAV in Chincoteague and Sinepuxent bays was located along the eastern sides of both bays behind Assateague Island, the barrier island that forms these bays. Several beds were located along the eastern side of Isle of Wight and Assawoman bays, behind Fenwick Island, the barrier island which forms these bays.