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The Po and Susquehanna rivers are the primary sources of freshwater and anthropogenic nutrients to both systems and each accounts for about half of total riverine inputs of nitrogen and phosphorus. Thus, the distributions of salinity and nutrients within the NAS and mainstem of CB reflect inputs of freshwater and nutrients by the Po (NAS) and Susquehanna (CB) rivers and internal dynamics (circulation and biogeochemical processes).

The two systems also differ in their bathymetry. The mean depth of CB is 8.4 m but 33.5 m in the NAS (Table 1.1). Chesapeake Bay is characterized by a shallow (< 10 m) soft bottom that flanks a relatively deep (10–50 m) central channel running the length of the oligohaline (salinity < 5) and mesohaline (salinity 5–18) reaches of the bay. The NAS is a continental shelf sub‐basin of the Adriatic Sea, the southern border of which is defined by the 100 m isobath (Poulain et al., 2001).

Figure 1.1 (Left) The mainstem Chesapeake Bay (CB), which runs 320 km from the mouth of the Susquehanna River to the Atlantic Ocean. (Right) The northern Adriatic Sea (NAS) extends 135 km southeast to the 100 m isobath. The Po is the largest river discharging into the NAS.

(Poulain et al. 2001. Reproduced with permission of Springer Nature).

The geomorphology of each of these two systems produces circulation patterns that increase the residence time of riverine inputs of nutrients and, as a consequence, promotes high levels of phytoplankton production and eutrophication in both systems. At the same time, given the large volume of the NAS relative to the input of nutrient‐rich Po River water (14 to 1) and the low volume of CB relative to the input of nutrient‐rich Susquehanna River water (1.4 to 1), CB is much more susceptible to eutrophication than is the NAS.

As in many coastal ecosystems, increases in land‐based nutrient inputs reflect increasing population density and industrial agriculture in their respective watersheds during the course of the Anthropocene (Meybeck, 2003). Although the annual mean volume transported by each of the two rivers is similar and their watersheds are roughly equivalent in area, total nitrogen (TN) and total phosphorus (TP) loads to the NAS are two to three times higher than to CB (Table 1.1). Differences in nutrient input reflect differences in population density (Po watershed ~230 people km⁻², Susquehanna ~60 people km⁻²), areal extent of agricultural development (Po agriculture ~60% of the watershed, Susquehanna agriculture ~30%), and levels of waste‐water treatment (more effective P removal in CB). Nonpoint N loads are greater than point sources for both systems, although point sources contribute a greater proportion of TN to the NAS (40%) than to CB (8–28%). The TP load for CB is largely (66%) from nonpoint sources. In comparison, 44–67% of the TP load for the NAS is from point sources.

Nutrient inputs are distributed via currents and mixing among water masses. The mean (residual) circulation of both systems is driven by thermohaline processes, but on scales of days to months this buoyancy‐driven seasonal flow is significantly modulated by wind shear (extratropical and tropical storms in CB and boras (cold, dry, katabatic northeast winds that occur most frequently during winter) and siroccos (warm, humid southerly winds that occur most frequently during summer) in the NAS). Differences in spatial dimensions are reflected in the circulation patterns of the two systems, which in turn affect the distributions of nutrients and phytoplankton biomass in both systems.

Since the width of CB is less than the Rossby radius of deformation (the length scale at which rotational effects become as important as thermohaline processes in determining patterns of circulation), lateral gradients in salinity and density are small relative to gradients along its north–south axis and prevailing flows tend to parallel the main channel. Thus, the primary mechanism of nutrient transport is a two‐layered, gravitational circulation with seaward flow of the surface layer and landward flow of the bottom layer. In contrast, the width of the NAS exceeds the Rossby radius so lateral (east–west) gradients promote the development of basin wide cyclonic gyres (Figure 1.1). Due to river runoff and heating in the late spring and summer and to autumn–winter cooling, gradient currents are established. This geostrophic response leads to a cyclonic gyre, with a NW current flowing into the NAS along the eastern margin and a SW current flowing out of the NAS along the western margin. The former transports warmer and saltier water into the NAS while the latter transports cooler, fresher water out of it. In late fall and winter, the nutrient‐rich Po outflow hugs the Italian coast forming a coastal boundary layer of buoyant water when most of the basin is vertically mixed. As the seasonal pycnocline sets up during spring and summer, the Po outflow tends to spread eastward across the basin resulting in longer residence times of nutrients.

Table 1.1 Mean riverine inputs and physical and ecological characteristics of CB and the NAS

Riverborne inputs	Po River	Susquehanna River
Freshwater input (km3 year−1)	46	36
Total N input (106 kg year−1)	164	63
Mean total P input (106 kg year−1)	8.8	2.8
Mean NOx input (106 kg year−1)	105	43
Mean dissolved inorganic P input (106 kg year−1)	3.0	0.4
Physical characteristics	NAS	CB
Length × mean width (km)	135 × 135	320 × 20
Mean depth (m)	33.5	8.4
Surface area (km2)	18,900	6500
Volume (km3)	635	50
Surface area/volume (km−1)	30	130
Dominant circulation pattern	Cyclonic gyres	Partially stratified, estuarine
Euphotic zone depth (m)	10–55	3–10
Phytoplankton	NAS	CB
Mean surface chlorophyll‐a (μg L−1)	1.5	9.5
Mean phytoplankton production (g C m−2 year−1)	90	450

Note: NO_x = dissolved nitrate + nitrite.

The parameters of nutrient recycling via pelagic–benthic interactions include net phytoplankton production, deposition of particulate organic matter (POM), benthic respiration, and benthic nutrient regeneration. While phytoplankton production is fivefold higher in CB than in the NAS, benthic community carbon respiration rates are fourfold higher in CB than in the NAS and differences in sediment nitrogen recycling rates (as NH₄⁺) are sevenfold to 15‐fold higher in CB. At the same time, rates of denitrification are nearly identical in the two systems. Thus, when normalized to the rate of respiration, the rate of denitrification (and potential loss of nitrogen to the atmosphere via the release of nitrous oxide and dinitrogen gas) is much higher in the NAS than in CB.