The effects of restoring a tidal regime in reclaimed areas

The  effects  of  restoring  a  tidal  regime  in  reclaimed   areas  

John  Bastiaan   S2385120   BSc  Thesis   Supervisors:  

Tjeerd  Bouma   Jim  van  Belzen  

Table  of  contents  

Abstract  ...  3    

Introduction  ...  3    

The  effects  on  abiotic  components  ...  4    

The  effects  on  biotic  components  ...  7    

Discussion  ...  8    

References  ...  9  

Abstract  

For   centuries,   salt   marshes   were   reclaimed   for   agricultural   or   urban   use,   destroying   biodiversity  with  it.  In  the  present,  people  have  begun  restoring  the  tidal  regime  in  these   reclaimed  areas.  This  seems  as  easy  as  breaching  a  dike  and  be  done  with  it,  but  it  is  a  bit   more  complicated  than  that.  This  thesis  looks  at  three  different  aspects  that  should  be   taken  into  account  when  reconstructing  a  salt  marsh:  the  geomorphology,  the  flora  and   fauna  and  the  geochemistry.  Little  seems  to  be  affecting  the  geomorphology  of  an  area,   for  the  shape  of  the  former  salt  marsh  remains  in  the  area.  The  flora  and  fauna  change,   because  many  fresh  water  plants  and  animals  cannot  cope  with  the  salinity  of  the  area.  A   saltmarsh  also  seems  to  function  like  a  sink  for  heavy  metals  and  nutrients,  which  come   from  agricultural  facilities  or  urban  districts.  The  biological  consequences  seem  positive   for   biodiversity,   but   economic   and   political   consequences   have   not   been   taken   into   account.  

Introduction    

Many  centuries  ago,  the  world  looked  a  lot  more  natural  than  it  does  now.  Rainforests   were   not   threatened   by   logging,   seas   were   not   polluted,   coral   reefs   flourished   in   the   tropical  oceans.  Trawlers  did  not  plough  the  bottom  of  the  sea  and  schools  of  fish,  the   size  we  have  never  seen,  roamed  the  oceans.  The  same  goes  for  intertidal  areas.  Many   natural  intertidal  areas  consisted  of  mudflats,  mangroves  or  salt  marshes.  Due  to  human   engineering,  by  building  dikes  and  dams,  we  cut  off  most  intertidal  areas  from  the  sea,   destroying   life   with   it  

(Lee  &  An,  2015).  Diked   salt  marshes  were  made   into   fresh   water   areas,   mostly   for   agricultural   purposes.   Especially   in   the   1970s,   everything   we   thought   about   was   progression,   neglecting   nature  and  the  impact  it   might   have   on   the   (local)   climate   (Fig.   1).  

People   thought   about   wetlands  like  they  were   wastelands,   good   for   absolutely  nothing.  Now   we   are   starting   to   feel   the   consequences   of  

years  of  polluting  and  transforming  nature  for  our  own  purposes  (Daily  mail  reporter,   2013).  

From  the  1990s  on,  people  saw  the  consequences  of  what  had  been  done  in  the  1970s   and  before  and  saw  that  change  was  necessary.  Ideas  began  to  develop  about  returning   those   transformed   areas   into   their   original   state   again.   Many   of   those   plans   were  

Figure  1:  Massive  mud  plumes  in  the  1970s  into  Mobile  Bay,  Alabama  U.S.  

Causing  grass  meadows  to  disappear.

actually   executed   in   the   1990s,   resulting   in   (partially)   restored   areas   (French,   2006;  

Wolters   et   al.   2005).   Dealing   with   coastal   erosion   is   also   an   important   reason   for   restoring   tidal   regimes   (Zedler,   2004).   In   this   thesis   restoration   of   land   reclamation   areas  is  the  main  theme.  Breaching  dikes  is  a  common  way  of  restoring  a  tidal  regime  in   an  area.  However,  using  this  method,  erosion  is  likely  to  take  place,  especially  if  currents   are   strong   or   the   area   is   windy.   A   way   to   (partially)   prevent   strong   currents   from   eroding  the  area  is  to  build  a  water  inlet,  which  lets  people  control  the  amount  of  water   inflow  in  an  area,  as  is  the  case  in  the  restored  salt  marsh  Rammegors  in  the  Netherlands   (Balke  et  al.  2014).    

Restoring   tidal   regimes   in   transformed   areas   should   change   the   environment   on   multiple  levels.  Geochemistry  is  likely  to  change  (Langis  et  al.  1991),  other  plants  will  be   growing   in   the   area   (Craft   et  al.   1999),   different   animals   will   occur   in   a   restored   area   (Thiet   et   al.   2014)   and   even   the   geomorphic   components   are   likely   to   change   (Hood,   2014).     These   components   might,   however,   vary   between   natural   salt   marshes   and   restored  salt  marshes  (Langis  et  al.  1991).  Another  component,  which  might  accelerate   or  inhibit  establishment  of  species,  is  elevation.  A  higher  elevation  might  develop  faster   than  an  almost  flat  area  (Brooks  et  al.  2015).  Coastal  protection  could  also  play  a  role  in   a  decision  to  restore  a  tidal  regime  in  an  area.  Plants,  like  Spartina  alterniflora,  stabilize   the  soil  and  keep  the  otherwise  loose  sediments  together  (Seneca  et  al.  1985).  

Looking   at   different   aspects,   I   want   to   know   what   the   consequences   of   restoring   tidal   regimes   in   fresh   water   areas,   including   agricultural   areas,   with   different   elevation   patterns   are.   I   will   compare   different   components   of   these   areas:   abiotic   factors,   including   geomorphic   and   geochemical   components,   and   biotic   factors.   The   difference   between   natural   salt   marshes   and   restored   salt   marshes   would   also   be   an   interesting   feature   to   look   at.   I   expect   there   will   be   differences   in   these   features.   Geochemistry   might  be  more  sulphide  based,  vegetation  will  shift  from  reed  areas  to  salt  marsh  areas,   grazing   animals   will   move   away   and   make   place   for   benthic   communities.   The   geomorphology  might  be  influenced  by  erosion  and  sedimentation.  

I   shall   point   these   different   characteristics   out   in   2   different   sections   describing   the   abiotic  and  biotic  characteristics  of  restored  areas.  Ultimately,  the  main  question  will  be   answered  in  the  discussion.  

The  effects  on  abiotic  components  

Geomorphology  

The  soil  of  salt  marshes  consists  mostly  of  clay,  deposited  by  the  sea  during  a  high  tide.  

In  the  past  people  reclaimed  salt  marshes,  because  of  the  fertility  of  the  soil,  which  was   one  of  the  reasons  of  global  decline  in  salt  marshes  (salt  mining  and  the  presence  of   fossil  fuels  were  also  reasons  for  use  of  these  areas)  (Costa  et  al.  2009).  When  a  salt   marsh  is  diked  off,  not  every  area  is  used  by  farmers,  because  some  areas  are  not  

suitable  for  farming,  leaving  these  bits  of  land  to  nature.  Peat  land  begins  to  develop  and   changes  several  components  of  the  soil,  like  soil  density  and  soil  chemistry.  However,   several  characteristics  of  the  area,  like  the  channel  networks,  remain  in  the  landscape.  

So  when  the  tide  is  restored  again  in  the  area,  the  gullies  are  on  the  same  places  than  

they  were  in  the  past  (Hood,  2014)  (Fig  2).  

There  are  however  several  factors  that  should  be  taken  into   account  when  restoring  a  tidal  regime  in  an  area.  Opening   an  area  sounds  simple  enough,  but   local  people  might  not   agree  with  breaching  a  dike  and  flooding  the  area,  because   they   will   lose   money   by   losing   it.   Culture   and   historic   events   could   also   affect   their   point   of   view.   In   1953,   for   example,   hundreds   of   people   died   by   a   dike   breach   in   Zeeland   in   the   Netherlands.   When   they   have   left,   a   dike   should   be   breached   in   order   to   restore   the   tidal   regime.  

Breaching  seems  the  easiest  option,  but  often  involves  a  lot   of  money,  so  the  use  of  a  water  inlet  is  very  common.  Some   doubt  however  the  sufficiency  of  partially  ‘breaching’  dikes,   because   it   would   not   restore   the   tidal   marsh   completely   (French  and  Stoddart,  1992).    Others  say  it  would  be  wise   to  not  breach  dikes  completely  in  order  to  protect  the  area   from  excessive  wave  action  (Weishar  et  al.  2001).  So  which   approach  is  most  optimal  stays  quite  unclear.  

The  main  actor  on  the  geomorphic  components  is  the  sea  in   this   area,   and   the   sea   acts   in   this   case   via   the   channel.  

Sediments  are  moved  and  deposited  by  the  channel  (French  and  Stoddart,  1992),  seeds   arrive   due   to   the   channel   (Sanderson   et   al.   2000)   and   fauna   arrives   via   the   channel   (Levy  and  Northcote,  1982).  When  an  area  is  restored,  the  sea  will  flow  in  at  the  lowest   point,  filling  up  the  old  gully  at  first.  The  current  is  strongest  at  the  water  inlet  or  the   dike  breach  and  diminishes  further  from  the  inlet.  So  the  soil  consists  mostly  of  sand  (or   rocks)  at  the  lower  points,  and  clay  or  silt  at  the  higher  locations,  which  is  also  the  case   in  natural  salt  marshes.  

The  geomorphic  components  do  not  seem  to  differ  much  between  natural  salt  marshes   and  reconstructed  ones.  If  we  look  at  different  elevations  however,  we  can  see  that  less   sediment  is  deposited  when  further  away  from  the  sedimentation  source  (Butzeck  et  al.  

2015).   Higher   areas,   which   lay   further   from   the   gully,   are   inundated   less   often   than   lower   areas,   so   less   sediment   can   be   deposited   in   higher   areas,   making   them   more   vulnerable   to   erosion   due   to   wind   or   rain,   especially   if   elevations   are   steeper   (water   rushes   much   faster   downwards   with   steeper   elevations).   There   are   however   no   significant   differences   between   sedimentation   rates   in   natural   salt   marshes   and   reconstructed  salt  marshes,  unless  the  elevation  is  very  low.  

Geochemistry  

One   aspect   that   has   not   been   described   is   the   geochemistry   in   a   restored   salt   marsh.  

Areas  that  are  converted  to  tidal  marshes  often  have  agricultural  histories.  The  problem   with   former   polders   is   that   they   are   often   contaminated   with   all   kinds   of   metals   that   entered  the  area  over  a  certain  period  of  time  (Teuchies  et  al.  2012).  Eutrophication  is   also   a   problem   (mainly   for   estuaries),   caused   by   increased   fluxes   of   nitrogen   and   phosphorus  due  to  human  development  (Nixon,  1995).  Both  problems  seem  to  originate   from  the  agricultural  sector,  which  uses  fertilizers  excessively  and  finding  solutions  to  

Figure  2:  Rammegors  area  before   restoring  of  tidal  regime.  The  pattern   of  the  gully  is  still  visible  in  the   landscape  after  reclaimation.  Being  a   sand  deposit,  the  gullies  still  

remained  in  the  landscape.

lessen  the  use  of  these  fertilizers  are  often  difficult  and  slow  processes  (Carstensen  et  al.  

2006).  In  order  to  inhibit  coastal  eutrophication,  salt  marshes  are  being  constructed  and   developed  as  they  reduce  these  nutrients,  as  well  as  increasing  biodiversity  and  carbon   sequestration.   In   this   section   I   will   describe   the   geochemical   characteristics   of   salt   marshes.  

Before   restoration   of   a   salt   marsh,   an   area   is   often   used   for   agricultural   purposes.  

Farmers  use  fertilizers,  herbicides  and  pesticides  to  optimize  their  production,  which  is   useful   for   the   farmer,   but   devastating   for   biodiversity.   These   agricultural   help   sources   wash   down   into   the   groundwater,   polluting   it.   Heavy   metals   from   pesticides   and   herbicides,  which  have  a  high  affinity  for  sediment  particles,  stay  in  the  soil,  remaining   there  for  years  (Teuchies  et  al.  2012).  Fertilizers  eutrophicate  the  groundwater,  causing   algal   blooms   in   fresh   water   sources,   destroying   biodiversity.   Ultimately,   the   groundwater   arrives   at   an   estuary,   eutrophicating   that   area   as   well.   Reconstructing   a   salt  marsh  might  solve  this  problem  (Nowicki  et  al.  1999)  .  

The  European  Parliament  has  implemented  the  Water  Framework  Directive,  which  aims   to   improve   the   ecological   status   of   aquatic   ecosystems   by   decreasing   the   amount   of   eutrophication  (Calvo-­‐Cubero  et  al.  2014).  One  of  the  solutions  to  reduce  eutrophication   is   to   reconstruct   salt   marshes.   Without   salt   marshes,   runoff   from   polders   will   enter   estuaries  and  pollute  these  areas.  But  when  salt  marshes  are  present,  the  estuaries  seem   much   more   clean.   This   difference   can   be   explained.   Salt   marsh   plant   species   have   the   ability   of   removing   toxic   components   from   the   soil,   such   as   heavy   metals   and   pesticides/herbicides  (Teuchies  et  al.  2012).  Another  characteristic  of  salt  marsh  species   is  to  remove  excessive  nutrients  from  the  soil,  which  originate  in  urban  and  agricultural   areas  (Busnardo  et  al.  1992).  However,  in  some  areas,  like  the  Ebro  delta,  this  ability  is   not  enough,  as  silicate  is  the  limiting  factor.  The  Ebro  River  has  a  series  of  170  dams  for   hydroelectric   power   supply.   Sediments,   which   contain   silicate,   are   trapped   behind   the   dams.   Nitrate   and   phosphorus   are   the   dominant   nutrients   in   the   delta,   and   cause   dinoflagellates   to   bloom,   some   causing   Harmful   Algal   Blooms   (HABs)   (Humborg   et  al.  

2000).    

In  cases  were  the  river  is  not  dammed  or  obstructed  in  another  way,  salt  marshes  seem   to   diminish   the   effects   of   eutrophication,   prohibiting   algal   blooms   and   pollution   of   estuaries.  So  reconstructing  salt  marshes  is  a  serious  solution  to  solving  eutrophication.  

Reducing   the   amount   of   fertilizers,   pesticides   and   herbicides   is   also   a   solution   to   reducing  pollution.    

Benthic   fauna,   or   so-­‐called   ecosystem   engineers,  is  also  able  to  change  some  aspects   of   the   soil,   especially   if   it   comes   to   oxygen   concentrations.   When   a   polder   is   flooded,   water  fills  the  pores  in  the  soil.  When  the  soil  is   sandy,   water   will   flush   down   to   the   groundwater.   But   when   the   soil   consists   of   clay,  water  sticks  to  the  clay  particles,  creating   a  film  of  water  around  every  clay  particle  and   blocking  oxygen  from  entering  the  soil.  Benthic   fauna,   like   lugworms,   plough   the   benthos   and  

therefore   aerating   the   soil,   making   it   suitable   for   salt   marsh   plant   species   (Thiet   et  al.  

2014).  

The  effects  on  plants  and  animals    

Although   the   geomorphic   components   might   not   change   much   by   returning   a   fresh   water  area  to  the  sea,  it  certainly  does  for  plants  and  animals.  When  seawater  starts  to   flush   into   the   area,   the   old   fresh   water   vegetation   will   die   off   and   make   place   for   salt   marsh  vegetation.  The  animals  living  in  the  former  fresh  water  area  start  moving  away,   because  of  lack  of  food  and  water,  and  will  make  place  for  animals,  which  thrive  in  salt   marsh  areas,  especially  molluscan  species  (Thiet  et  al.  2014).    In  this  section  we  will  look   more  closely  to  the  effects  of  restoring  tidal  regimes  in  fresh  water  areas  on  flora  and   fauna.  

Salt  marsh  vegetation  and  fresh  water  vegetation  differ  in  physiology.  Salt  marsh  plants   have  different  ways  of  handling  the  salty  water.  Some  plants  excrete  salt  on  their  leaves,   other  store  the  salt  in  special  developed  pockets.  Fresh  water  plants  do  not  have  these   characteristics  and  are  therefore  rapidly  outcompeted  by  salt  marsh  vegetation.  

So  when  a  fresh  water  area  is  exposed  to  the  sea,  most  of  the  fresh  water  vegetation  will   die,  but  not  directly  be  replenished  by  salt  marsh  vegetation.  Dying  vegetation  will  result   in   a   lot   of   decomposition   in   the   area,   causing   a   terrible   stench   in   the   area,   which   is   a   nuisance  for  people  inhabiting  the  area  around  the  salt  marsh.  Dying  plants  cause  soil   enrichment  as  well,  as  will  be  discussed  in  the  following  section.  A  problem  that  follows   is   the   establishment   of   new   salt   marsh   species   in   the   area,   which   need   a   so   called   Window  of  Opportunity  to  establish  (Balke  et  al.  2014).  A  Window  of  Opportunity  is  a   certain  period,  in  which  the  conditions  are  optimal  for  establishing.  In  salt  marshes  this   period  is  often  referred  to  a  timeframe  in  which  the  area  is  not  inundated  for  a  while.  

But   inundation   is   a   necessity   for   most   tidal   marsh   vegetation   (there   are   a   few   exceptions),  as  their  seeds  must  be  spread  by  the  water.  After  being  carried  to  a  location,   a   seeds   needs   to   establish,   which   requires   a   minimal   amount   of   disturbance.   When   a   seed  has  established,  the  plant  can  focus  on  growing  and  reproducing  itself.  

Change  is  also  inevitable  when  it  comes  to  fauna  in  the  area.  Before  an  area  is  inundated,   grazing  animals,  like  deer,  geese  and  rabbits,  are  likely  to  inhabit  the  area.  When  an  area   is   inundated,   their   food   and   water   sources  

disappear,   so   they   have   to   leave   after   a   certain   period   of   time.   The   animals   that   will   populate   the  new  environment  must  be  adapted  to  highly   variable   conditions,   especially   the   benthic   invertebrates   (Thiet   et  al.   2014).   However,   they   provide   an   essential   role   in   any   intertidal   zone:  

they   provide   nutrients   for   plants   by   consuming   detritus  and  phytoplankton  and  are  an  excellent   food   source   for   many   coastal   bird   species,   like   the   spoonbill   in   Figure   3,   making   salt   marshes   ideal  foraging  regions  (Prins  et  al.  1998).  If  a  lot  

of  vegetation  dies  off  however,  the  soil  does  not  get  oxygenated,  resulting  often  in  the   area   becoming   hostile   to   any   organism   (except   the   obligate   anoxic   prokaryotes).   So   before  flooding  a  fresh  water  area,  precautionary  measures  are  taken,  such  as  removing   trees   from   the   area   and   mowing   the   grass/reeds.   When   these   measures   have   been   taken,  less  decomposition  will  take  place  when  the  area  is  flooded,  leaving  more  space   for  salt  marsh  vegetation  to  develop  and  benthic  fauna  to  thrive.  

Discussion      

We  looked  at  three  different  aspects  that  are  influenced  by  restoring  a  tidal  regime  in  a   fresh  water  area:  geomorphology,  flora,  fauna  and  geochemistry.  These  three  factors  are   important   for   determining   the   structure   of   a   salt   marsh.   Many   of   the   natural   salt   marshes  were  reclaimed  in  the  19^th  and  the  20^th  century,  losing  a  lot  of  biodiversity  in   the  process.  The  areas  that  were  leftover  had  a  hard  time,  especially  in  the  1960s  and   1970s  (Lee  &  An,  2015).  Seeing  what  ‘progression’  had  done  to  nature,  restoration  plans   started  to  develop.  In  the  present,  many  plans  have  been  executed  and  many  sites  have   been  reconstructed  worldwide.    

In  this  thesis,  I  wanted  to  look  at  what  the  consequences  are  of  returning  former  fresh   water   areas   to   the   sea.   I   looked   at   three   different   aspects   of   a   salt   marsh,   mentioned   above.  The  geomorphology  is  one  aspect  of  a  salt  marsh.  When  the  salt  marsh  area  was   reclaimed,   the   shape   of   the   gully   remained   in   the   landscape   and,   when   returned,   the   gully   is   still   the   same   as   it   was   before.   Unless   people   do   not   take   precautionary   measures,   like   regulating   the   amount   of   water   inflow   in   the   area,   the   geomorphology   will  not  change  much.  

Another  aspect  looked  at  was  flora  and  fauna.  When  restoring  a  tidal  regime  it  is  quite   obvious  that  the  flora  will  change,  as  most  plants  do  not  have  much  resistance  against   salty   conditions.   However,   when   an   area   is   suddenly   exposed   to   the   sea,   the   newly   arrived   salt   marsh   species   will   have   to   settle,   which   is   a   difficult   effort.   Some   basal   infrastructure,   as   mentioned   in   the   geomorphological   part,   is   necessary   for   seeds   to   arrive  and  become  seedlings.  The  fauna  changes  as  well,  from  large  mammals,  like  deer,   to   benthic   invertebrates,   like   lugworms.   The   regime   of   birds   does   not   seem   to   change   much  however,  as  the  restored  salt  marsh  was  already  in  a  coastal  area.  

Ultimately,   I   looked   at   geochemistry   as   a   factor   in   the   restored   salt   marsh.   Most   salt   marshes   have   been   agricultural   or   urban   areas   in   the   past   and   are   therefore   polluted   with   heavy   metals   or   eutrophicated.   The   runoff   will   arrive   in   an   estuary,   which   will   become  polluted  as  well,  often  resulting  in  HABs.  So  restoring  a  tidal  regime  often  has   economical   and   political   reasons   as   well,   preventing   loss   of   biodiversity   and   loss   of   income.   Salt   marsh   plant   species   absorb   heavy   metals   and   help   reducing   the   level   of   nitrogen  and  phosphorus,  the  main  actors  in  eutrophication,  if  the  silicate  levels  are  high   enough.  Benthic  fauna,  like  the  lugworm  Arenicola  marina,  also  help  oxygenate  the  soil,   creating  more  suitable  areas  for  salt  marsh  vegetation  to  grow.  

The   main   question   was   what   the   consequences   are   of   turning   fresh   water   areas   into   areas   where   the   tidal   regime   is   restored.   Returning   these   areas   to   the   sea   improves  

biodiversity  and  biogeochemistry.  Some  fresh  water  species  might  be  lost  in  some  cases,   but   other   rare   species   return   in   their   place.   Geomorphology   is   not   much   affected   by   restoring  a  tidal  regime,  but  is  very  important  for  salt  marsh  species  to  establish.  This   thesis  focused  mostly  on  biological  aspects,  but  has  not  been  very  elaborate  about  the   economic  or  political  aspects.  So  it  might  be  interesting  to  look  at  those  aspects  as  well   in  another  study.    

We  should  certainly  continue  to  develop  new  salt  marsh  areas  and  restoring  more  tidal   regimes  in  every  part  of  the  world,  especially  in  Asia,  where  biodiversity  keeps  declining   (Sno   et  al.  2015).   A   lot   of   work   still   needs   to   be   done,   but   at   least   some   progress   has   been  made  we  can  proudly  look  upon.  

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-­‐ http://www.dailymail.co.uk/news/article-­‐2291247/Polluted-­‐America-­‐1970s-­‐

green-­‐movement-­‐grows.html  (Consulted  on  15  June  2015)