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Salt marsh
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===Urban development and nitrogen loading=== [[File:Chaetomorpha linum-salt marsh algae.jpg|thumb|''[[Chaetomorpha|Chaetomorpha linum]]'' is a common [[marine algae]] found in the salt marsh.]] The conversion of marshland to upland for agriculture has in the past century been overshadowed by conversion for urban development. Coastal cities worldwide have encroached onto former salt marshes and in the U.S. the growth of cities looked to salt marshes for waste disposal sites. Estuarine pollution from organic, inorganic, and toxic substances from urban development or industrialisation is a worldwide problem<ref name="long"/> and the sediment in salt marshes may entrain this pollution with toxic effects on floral and faunal species.<ref name="ranwell"/> Urban development of salt marshes has slowed since about 1970 owing to growing awareness by environmental groups that they provide beneficial [[ecosystem services]].<ref name="bromberg"/> They are highly productive [[ecosystem]]s, and when net productivity is measured in g m<sup>โ2</sup> yr<sup>โ1</sup> they are equalled only by tropical rainforests.<ref name="long"/> Additionally, they can help reduce wave erosion on sea walls designed to protect low-lying areas of land from wave erosion.<ref name="boorman"/> De-naturalisation of the landward boundaries of salt marshes from urban or industrial encroachment can have negative effects. In the Avon-Heathcote estuary/Ihutai, New Zealand, [[species abundance]] and the physical properties of the surrounding margins were strongly linked, and the majority of salt marsh was found to be living along areas with natural margins in the Avon / ลtฤkaro and ลpฤwaho / Heathcote river outlets; conversely, artificial margins contained little marsh vegetation and restricted landward retreat.<ref name="jupp">Jupp, K. (2007). Establishing a physical and biological basis for salt marsh restoration and management in the Avon-Heathcote Estuary. Christchurch, University of Canterbury.</ref> The remaining marshes surrounding these urban areas are also under immense pressure from the human population as human-induced nitrogen enrichment enters these habitats. Nitrogen loading through human-use indirectly affects salt marshes causing shifts in vegetation structure and the invasion of non-native species.<ref name="bertness"/> Human impacts such as sewage, urban run-off, agricultural and industrial wastes are running into the marshes from nearby sources. Salt marshes are nitrogen limited<ref name="bertness"/><ref name="langis">{{Cite journal |last=Langis |first=Rene |last2=Zalejko |first2=Malgorzata |last3=Zedler |first3=Joy B. |date=1991-02-01 |title=Nitrogen Assessments in a Constructed and a Natural Salt Marsh of San Diego Bay |url=https://esajournals.onlinelibrary.wiley.com/doi/10.2307/1941846 |journal=Ecological Applications |language=en |volume=1 |issue=1 |pages=40โ51 |doi=10.2307/1941846 |issn=1051-0761|url-access=subscription }}</ref> and with an increasing level of nutrients entering the system from [[Human impact on the environment|anthropogenic effects]], the plant species associated with salt marshes are being restructured through change in competition.<ref name="bromberg"/> For example, the New England salt marsh is experiencing a shift in vegetation structure where ''S. alterniflora'' is spreading from the lower marsh where it predominately resides up into the upper marsh zone.<ref name="bertness"/> Additionally, in the same marshes, the reed ''[[Phragmites australis]]'' has been invading the area expanding to lower marshes and becoming a dominant species. ''P. australis'' is an aggressive halophyte that can invade disturbed areas in large numbers outcompeting native plants.<ref name="bromberg"/><ref name="chambers">{{Cite journal |last=Chambers |first=Randolph M. |last2=Meyerson |first2=Laura A. |last3=Saltonstall |first3=Kristin |date=September 1999 |title=Expansion of Phragmites australis into tidal wetlands of North America |url=https://linkinghub.elsevier.com/retrieve/pii/S0304377099000558 |journal=Aquatic Botany |language=en |volume=64 |issue=3-4 |pages=261โ273 |doi=10.1016/S0304-3770(99)00055-8|url-access=subscription }}</ref><ref name="warren">{{Cite journal |last=Warren |first=R. Scott |last2=Fell |first2=Paul E. |last3=Rozsa |first3=Ron |last4=Brawley |first4=A. Hunter |last5=Orsted |first5=Amanda C. |last6=Olson |first6=Eric T. |last7=Swamy |first7=Varun |last8=Niering |first8=William A. |date=2002-08-20 |title=Salt Marsh Restoration in Connecticut: 20 Years of Science and Management |url=https://onlinelibrary.wiley.com/doi/10.1046/j.1526-100X.2002.01031.x |journal=Restoration Ecology |language=en |volume=10 |issue=3 |pages=497โ513 |doi=10.1046/j.1526-100X.2002.01031.x |issn=1061-2971|url-access=subscription }}</ref> This loss in biodiversity is not only seen in flora assemblages but also in many animals such as insects and birds as their habitat and food resources are altered.
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