The distribution of submerged aquatic vegetation (SAV), principally rooted vascular macrophytes, in the Chesapeake Bay, its tributaries, and the Chincoteague Bay section, was mapped from black-and-white aerial photographs taken during May to October 1995 at a scale of 1:24,000. SAV bed perimeter information was digitized and stored in a computerized database.
SAV distribution data in this report are presented and discussed based on the segmentation scheme adopted by the Chesapeake Bay Program (Flemer et al., 1983) and described briefly in the Methods section of this report. This 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). The 1995 data were edgematched using ARC/INFO GIS software, as were all the historical SAV bed 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, 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).
SAV in the Chesapeake Bay decreased 8% in 1995 to a total of 24,251.57 hectares mapped from aerial photography, a 2,232.81 hectare decline from the 26,484.38 hectares mapped in 1994 (Figure 1; VIMS SAV GIS Database). This follows a 10% decrease in 1994 from the 29,587.47 hectares in 1993 and is the lowest level since the 1989 level of 24,151.49 hectares (VIMS SAV GIS Database). SAV also decreased in all three zones of the Chesapeake Bay in 1995 (14%, 8%, and 7% in the Upper, Middle, and Lower zones, respectively) compared to 1994 when SAV increased only in the Upper zone (44%) and decreased in the Middle and Lower zones 21% and 8%, respectively, from totals in 1993 (Figure 2; Orth et al., 1995; VIMS SAV GIS Database). In 1995, SAV decreased 529.61 hectares in the Upper zone, with a total of 3,324.21 hectares (14% of the Bay total) mapped, compared with 3,853.82 hectares (15% of the Bay total) mapped in 1994 (Figure 2). SAV decreased 1,096.84 hectares in the Middle zone, with 12,276.42 hectares (51% of the Bay total) mapped in 1995, compared to 13,373.26 hectares (50% of the Bay total) mapped in 1994 (Figure 2). SAV decreased 606.36 hectares in the Lower zone, with 8,650.94 hectares (36% of the Bay total) mapped, compared to 9,257.30 hectares (35% of the Bay total) mapped in 1994 (Figure 2). SAV increased in only five segments in the Upper Bay zone in 1995, decreased in six (WT5 became unvegetated), and WT1 and WT4 remained unvegetated, compared to 1994, when SAV increased from 1993 levels in all segments except for WT1 and WT4 which remained unvegetated (Figure 3; Orth et al., 1995; VIMS SAV GIS Database). As in 1994, increases in 1995 for some segments of the Middle and Lower zones did not offset larger decreases in 1995 in other segments of these zones (Orth et al., 1995; VIMS SAV GIS Database). In the Middle Bay zone in 1995, there were increases in five segments, decreases in eight, and seven were unvegetated (six remained unvegetated from 1994) compared to 1994 when there were increases in six segments, decreases in ten, and six were unvegetated; and in the Lower Bay zone in 1995, there were increases in three segments, decreases in four, and six remained unvegetated from 1994, compared to 1994 when there were increases in three segments, decreases in four, and six were unvegetated (Figures 4 and 5; Orth et al., 1995).
In 1995, SAV in the Bay increased in thirteen segments (Upper zone: CB2, CB3, WT6, ET3, ET4; Middle zone: WT7, LE2, ET8, ET9, EE1; and Lower zone: LE4, LE5, WE4), compared to twenty segments in 1994 (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) (Orth et al., 1995). In 1995, SAV decreased in eighteen segments (Upper zone: CB1, WT2, WT3, WT5, ET1, ET2; Middle zone: CB5, WT8, TF1, RET1, TF2, RET2, EE2, EE3; Lower zone: CB6, CB7, CB8, LE3) compared to fourteen segments in 1994 (Middle zone: CB4, CB5, LE1, TF2, RET2, ET5, ET8, ET9, EE2, EE3; and Lower zone: CB6, CB7, LE3, WE4) (Orth et al., 1995). In 1995, sixteen were unvegetated (Upper zone: WT1, WT4, WT5; Middle zone: CB4, RET1, LE1, ET5, ET6, ET7, ET10; and Lower zone: TF3, RET3, TF4, RET4, TF5, RET5), compared to fourteen in 1994 (Upper zone: WT1, WT4; Middle zone: CB4, LE1, ET5, ET6, ET7, ET10; and Lower zone: TF3, RET3, TF4, RET4, TF5, RET5) (Orth et al., 1995). [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 devoid of submerged vegetation for years and are not currently photographed (Orth et al., 1995; VIMS SAV GIS Database).] There were no segments which were unvegetated in 1994 which had SAV detected by photography in 1995 (Orth et al., 1995; VIMS SAV GIS Database). Two segments (Upper zone: WT5; Middle zone: RET1) which were vegetated in 1994, albeit with relatively small amounts, had no SAV detected in 1995, compared to three segments (CB4, LE1, and ET5), all in the Middle zone, which were vegetated in 1993 but had no SAV detected in 1994 or 1995 (Orth et al., 1995; VIMS SAV GIS Database).
In the Bay in 1995, 44% of SAV was categorized as dense (density class 4, or 70-100% coverage), up slightly from the 1994 amount of 36% (Orth et al., 1995; VIMS SAV GIS Database). In 1995, 12% of SAV was categorized as moderate (density class 3, or 40-70% coverage), down slightly from the 1994 amount of 19% (Orth et al., 1995; VIMS SAV GIS Database). There was 26% of SAV categorized as sparse (density class 2, or 10-40% coverage), a decrease compared with the 1994 amount of 28%; whereas 17% was categorized as very sparse (density class 1, or 0-10% coverage), the same as in 1994 (Orth et al., 1995; VIMS SAV GIS Database). SAV in the very sparse and sparse classes combined (the 0-40% coverage range) constituted 44% of all SAV in 1995, a slight decrease from 45% in 1994 (Orth et al., 1995; VIMS SAV GIS Database). Conversely, SAV in the moderate and dense classes combined (the 40-100% coverage range) constituted 56% of all SAV in 1995, an slight increase from 55% in 1994 (Orth et al., 1995; VIMS SAV GIS Database).
The Chincoteague Bay section (consisting of Chincoteague, Assawoman, Sinepuxent, and Isle of Wight bays) had 3,758.29 hectares in 1995, with 15% classified dense, compared to 4,117.53 hectares in 1994, with 55% classified dense (Figure 2; Tables 5-9).