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pressures that are occurring globally (e.g., nutrient enrichment, fishing pressure, coastal development);

      2 global, climate‐driven pressures (e.g., ocean warming and acidification, sea‐level rise, acceleration of the water cycle, the El Niño–Southern Oscillation) that have local consequences.

      Together, local and global pressures interact to impact the capacity of marine ecosystems to provide services, especially in the coastal zone where these pressures converge and people and ecosystem services are concentrated (Elliff & Kikuchi, 2015; Solé & Ariza, 2019).

      The rationale for comparing the two coastal systems of the northern Adriatic Sea (NAS) and Chesapeake Bay (CB) is that: both are well studied, semi‐enclosed, river‐dominated ecosystems subject to anthropogenic nutrient enrichment; both provide similar, economically important services that have been subjected to similar pressures during the course of the Anthropocene (Zalasiewicz et al., 2010); and together they offer contrasts in geomorphology and circulation that modulate their responses to anthropogenic pressures (Cloern, 2001).

1.2. THE 1999 COMPARISON

      Malone et al. (1999) published a comparative analysis of the NAS and CB. Here, we provide a brief description of the watersheds of the two systems and a synopsis of that comparison.

      1.2.1. Overview of the Watersheds

Chesapeake Bay is a large estuary in the mid‐Atlantic region of the United States with a watershed of 166,530 km² that hosts about 18 million people. Among its many tributaries, the Susquehanna River accounts for ~50% of total riverine inputs of freshwater into CB and its tributaries and discharges directly into the head of the bay’s mainstem (Figure 1.1).

      The watershed of the Susquehanna is dominated by forested areas (~65%). Agricultural and developed lands account for ~35% of the watershed but much higher proportions of nutrient inputs to CB. Major physiographical provinces in the CB watershed include Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateau. Topographic conditions and soil permeability are variable within and among these regions. In general, steep slopes in the uplands of the Valley and Ridge contrast with gentle slopes of the Appalachian Plateau and Piedmont and flat terrain of the Coastal Plain. The ratio of watershed area to the volume of CB, an indicator of the extent to which land‐use practices in the watershed can impact water quality in the bay, is 1400 km⁻¹.

      The NAS is a semi‐enclosed arm of the northeast Mediterranean Sea with sandy coasts and alluvial plains to the north and west and rugged mountains to the east. The watershed spans 110,600 km² and hosts about 20 million people. The Po River, the largest river discharging into the NAS, has a watershed that comprises 67% of the total NAS watershed and hosts large urban and industrial settlements, as well as extended areas of intensive cropping and livestock activities. Its watershed is bounded by two mountain ranges, the Apennines and Alps. In contrast to the CB, ~62% is in agriculture and developed land with only 34% forested (Malagó et al., 2019). However, with a watershed‐area/seawater volume ratio of only 174 km⁻¹ (compared to 1400 km⁻¹ for CB), the potential impact of land‐use practices on the NAS as a whole is much smaller than that for CB.

      1.2.2. Nutrient Inputs, Circulation, and Nutrient Recycling

      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

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