3.1. Environmental Factors
Mean monthly water temperature in 2012 varied from 5°C in February to 25°C in July and then decreased to about 5 °C in December (Figure 2
a). In 2013, mean temperature was 30.7 °C in July and 7 °C in December. Water temperature was on average lower in 2012 than in 2013. Lower water temperatures were observed at sites close to the inlets, while St. Martin River had the highest mean water temperature (Table 1
Mean monthly salinity was higher in 2012 (33.4) than in 2013 (26.6) during the period of March to August, but the reverse occurred from September to November (Figure 2
b). This salinity pattern reflected the pattern of freshwater discharge by Birch Branch at Showell, MD (Figure 2
c). From January to July, Birch Branch mean discharge was higher in 2013 (0.45 m3
) than in 2012 (0.11 m3
), and the reverse was the case from August to November (Figure 2
c). Discharge was highest in October 2012, the month that Hurricane Sandy occurred in the area. Mean monthly salinity was between 23.6 and 34.7. Newport Bay had the lowest mean salinity level and Sinepuxent Bay had the highest salinity (Table 1
). Salinity was also lower at sites in St. Martin River and Assawoman Bay relative to sites in Chincoteague and Isle of Wight Bays.
Monthly dissolved oxygen (DO) levels on the average were high in fall and winter, but relatively lower in spring and summer, although dissolved oxygen levels were > 4.6 mg L−1
at all sites throughout the duration of this study (Figure 2
d). The lowest DO level was recorded in July (4.6 mg L−1
), and the highest in March (12.7 mg L−1
The seasonal patterns of chlorophyll a
concentrations are similar in 2012 and 2013. Monthly levels of chlorophyll a
averaged between 0.2 and 3.4 µg L−1
e). Chlorophyll a
levels were relatively high in winter but decreased in spring, particularly in April. Chlorophyll a
peaked in summer (July) in both years. Site 6 (Newport Bay) had the highest mean level of chlorophyll a
in the MCBs during this study (Table 1
). Monthly mean pH levels in the MCBs ranged from 7.8 to 8.2 and did not vary much within the system (Figure 2
3.2. Temporal Pattern of Mesozooplankton Abundance
Mesozooplankton abundance expressed as individuals m−3
) varied from 5 ind. m−3
(Site 12) in May 2012 to 410,509 ind. m−3
(Site 8) in September 2013. The average monthly abundance was high (24,543 ind. m−3
) in February, but peaked in early March (36,878 ind. m−3
) during winter of 2012; it declined to a relatively low level in spring and then gradually increased from summer through fall (Figure 3
). Relatively high abundance (30,032 ind. m−3
) of mesozooplankton was observed in December 2012. A gradual decline was also observed from winter 2013 through early spring. Densities remained low (378 ind. m−3
, May 2012) in spring and early summer, peaked (34,828 ind. m−3
) in September 2013 and then declined in fall.
Mesozooplankton abundance differed significantly between years (Table 2
) in summer and fall (p < 0.001), but not in winter and spring (p > 0.05). In fall, abundance was significantly higher in 2012 than 2013 (Mann–Whitney U = 261, T = 2040.0, p ≤ 0.001), but in summer, it was lower in 2012 than in 2013 (U = 392.0, T = 1909.0, p ≤ 0.001).
Seasonal and spatial patterns of abundance of mesozooplankton are presented in Figure 4
. For each of the seasons examined, there were no significant differences among sites in mesozooplankton abundance (ANOVA, p > 0.05) except in summer 2012 (ANOVA, p = 0.05) when the highest density was recorded (Figure 4
and Table 3
) at Site 7 (Tukey test, p < 0.05). When mesozooplankton abundance was compared among bays, higher abundance was recorded in Sinepuxent Bay than in any other bay in winter and summer 2012 (ANOVA, p < 0.05), as shown in Table 3
3.3. Community Composition, Relative Abundance and Distribution of Non-Copepod Mesozooplankton in MCBs
Mesozooplankton community in the MCBs was represented by 14 major taxa dominated by copepods, but only groups that are relatively abundant are presented in Figure 5
. Non-copepods contributed about 5% of the total mesozooplankton abundance. A detailed description of copepod species composition and abundance in the MCBs has been made [42
The non-copepod community on the average was dominated by hydromedusae (3%), followed by gastropod veliger (0.5%), and barnacle nauplii (0.5%) (Figure 5
). Cladocerans (0.1%) and larval stages of decapods (0.2%), polychaetes (0.2%), bivalves (0.1%), and fish (0.1%) were also relatively abundant within the non-copepod community. Larvaceans, nematodes, amphipods, and isopods made up <1% of the zooplankton community.
The percentage contribution of non-copepods to total zooplankton abundance was as high as 98% at Site 12 (AWB) in the summer when copepod relative abundance was low. Monthly averaged contribution varied between 0–31.9% in 2012 and 0.4–76.5% in 2013 (Figure 6
). Non-copepods were most represented in May 2012 and June 2013.
In 2012, barnacle nauplii were the most abundant non-copepods, with mean density of 810 ind. m−3
in Newport Bay (Figure 7
a,b). Annual peaks were observed in April 2012 (368 ind. m−3
) and September 2013 (102 ind. m−3
). The maximum barnacle nauplii density (4645 ind. m−3
) was attained in April 2012 in Newport Bay and mean abundance of barnacle nauplii was significantly higher in Newport Bay than in other embayments (ANOVA, p = 0.02). Although, barnacle nauplii were absent in fall 2012, they were present in all seasons in 2013 (Figure 8
a–d). Maximum density in 2013 was 1119 ind. m−3
in September at Site 3 (Chincoteague Bay). Barnacle nauplii were more abundant in spring and summer than in the fall and winter (ANOVA, p < 0.001).
In 2013, hydromedusae were the most abundant non-copepod mesozooplankton (Figure 7
c,d). Mean densities as high as 1367 ind. m−3
and 5156 ind. m−3
were recorded in Assawoman Bay and St. Martin River, respectively, in 2013. Hydromedusae were not observed at St. Martin River and Newport Bay sites in 2012 and also were not observed in Sinepuxent Bay in 2013 (Figure 7
b,d). They were present from March to August in 2012 and from March to September in 2013, but were not found in May 2013 (Figure 8
a–d). Average hydromedusae abundance was less than 20 ind. m−3
each month and at all sampling sites in 2012. Monthly mean densities peaked in April, 2012 (16 ind. m−3
) and in March 2013 (5252 ind. m−3
). The highest density recorded in 2013 (50,399 ind. m−3
, March, Site 10) was 298 times greater than the highest density (181 ind. m−3
, March, Site 5) in 2012.
Gastropod larvae were intermittently present and their monthly mean density reached a peak in May 2012 (158 no m−3
, Figure 8
a). A secondary peak (103 ind. m−3
) occurred in July of the same year. Mean density in 2013 peaked in June (721 ind. m−3
). Gastropod larvae were abundant (107 ind. m−3
) in Sinepuxent Bay, but were not observed in Newport Bay in 2012 (Figure 7
a–b). A maximum density (1299 ind. m−3
) of the larvae was observed in May 2012 at Site 7 (Sinepuxent). In 2013, maximum density (1852 ind. m−3
) occurred at Site 4 (Chincoteague Bay) in June. Gastropod larvae were least abundant in Assawoman Bay (2 ind. m−3
) in 2013.
Polychaete larvae occurred from February through August in 2012 and were most abundant (266 ind. m−3
) in March (Figure 8
a). Mean polychaete larval densities never exceeded 10 ind. m−3
in 2013, occurred sporadically, and were most abundant in March (9 ind. m−3
). Polychaete larvae were more abundant in 2012 than 2013 (Mann–Whitney U-test, p = 0.0002). Peak numbers were 2384 ind. m−3
in March 2012 at Site 10 located in St. Martin River and 60 ind. m−3
in March 2013 at Site 5 in Chincoteague Bay (Figure 7
Bivalve larvae were present from February through August in 2012 (Figure 8
a), but their occurrence was irregular in 2013 (Figure 8
c) with mean monthly densities < 5 ind. m−3
. Bivalve larvae were better represented in plankton samples from 2012 than 2013 (Mann–Whitney U-test, p = 0.001) and relatively abundant in March during both years (Figure 7
). Average density was 161 ind. m−3
in March 2012, and 4 ind. m−3
in March 2013. Maximum densities of 1490 ind. m−3
at Site 10 (St. Martin River) in 2012 and 58 ind. m−3
at Site 8 (Sinepuxent Bay) in 2013 were recorded in March. Bivalve larvae were only found in Sinepuxent Bay and Isle of Wight Bay (Site 9) in 2013, but their distribution in 2012 was widespread except at Sites 3 and 6 where they were not observed. Densities did not differ significantly amongst embayments (ANOVA, p = 0.12).
Cladocerans (Evadne nordmanni
, E. spinifera
, Pseudoevadne tergestina, Pleopis polyphemoides
, Podon intermedius
) were most abundant in Sinepuxent and Isle of Wight Bays (ANOVA, p = 0.002) during summer and were absent from Newport Bay (Figure 7
Decapod larvae (shrimps and crabs) were prevalent in Sinepuxent Bay where they occurred from March through November in 2012 and 2013 (Figure 7
; Figure 8
). In July, they comprised 42% and 67% of non-copepod zooplankton abundance, and 10% and 11% of total mesozooplankton abundance in 2012 and 2013, respectively. Larval stages of crabs Panopeus herbstii
, Neopanope texana
spp. and Pinnixia
spp. were observed in zooplankton samples collected in 2012. Zoeae of Callinectes sapidus
, Rhithropanopeus harrisii
, Ovalipes ocellatus
, Cancer irroratus
sp., Libinia dubia
, Ocydopode spp., Petrolisthes armatus
, and Lepidopa websteri
were also present.
Fish larvae were first observed in April 2012 (mean: 16 ind. m−3
) when they were most abundant (Figure 8
). They were observed in December 2013 (0.1 ind. m−3
), but mean densities were very low. The highest density of fish larvae (202 ind. m−3
) was observed at Site 6 in Newport Bay (April 2012).
Larvaceans occurred from April to June 2012 and in August 2013. Average densities ranged from 1 to 5 ind. m−3. The maximum density of larvaceans was 23 ind. m−3, at Site 8 (Sinepuxent Bay) in August, 2013. Other non-copepods irregularly found in samples included amphipods, isopods, nematodes, and ostracods.
3.5. Mesozooplankton Diversity
The diversity of mesozooplankton varied temporally and spatially and was affected by salinity such that diversity was higher at higher salinity than at lower salinity (Figure 10
). The Shannon–Wiener diversity index was high (≥2.5) in 2012 at all sites except at Site 6 (H′ = 2.2) in Newport Bay with the lowest diversity (Figure 10
). Diversity index value in Newport Bay (Site 6) in 2012 (H′ = 2.2) was slightly lower than in 2013 (H′ = 2.3). On the average, diversity was significantly higher (t = 6.2; p ≤ 0.001) in 2012 than in 2013. Sinepuxent Bay supported the highest diversity (H′ = 2.8 in 2012; H′ = 2.6 in 2013) of mesozooplankton in both years. Diversity decreased from Sinepuxent Bay (Sites 7 and 8) close to the Ocean City Inlet to Assawoman Bay (Site 13) in the northern MCBs in both 2012 and 2013. Likewise, diversity in Chincoteague Bay decreased from Site 5 to Sites 1 and 2 in both years.
Diversity was higher in spring and summer (H′ > 2.4) than in fall (H′ < 1.9) of 2012. In 2013, diversity in March and from September to December was higher (t = −2.3; p ≤ 0.06) than diversity in 2012. Periods of maximum and minimum diversity index values were different in 2012 and 2013. Mesozooplankton was most diverse in June (H′ = 2.9) and March (H′ = 2.5), but least diverse in October (H′ = 1.4) and July (H′ = 1.8) in 2012 and 2013, respectively.
When examined spatially, diversity indices values in 2012 were significantly correlated negatively with Chl. a (r = −0.7, p = 0.01) and positively with salinity (r = 0.7, p = 0.02), but not with But-fuco (r = −0.5, p = 0.08). In 2013, diversity also correlated positively with salinity (r = 0.3, p < 0.050). All other environmental factors were weakly correlated with mesozooplankton spatial diversity in 2013, and no significant relationship was observed (p > 0.05). With respect to monthly diversity, H′ was significantly correlated with ctenophores (r = 0.7, p = 0.02) in 2012, but not in 2013 (r = − 0.1, p = 0.9). Other variables such as temperature (r = − 0.4, p = 0.2) and salinity (r = − 0.4, p = 0.3) showed weak negative correlations with diversity in 2013.